DE102008032532B4 - Method and device for preparatory laser material removal - Google Patents

Abstract

Method for preparatory laser material removal for carrying out laser emission spectrometry, in which
- a surface of an object is irradiated with one or more first laser pulses to remove a surface layer, and
- Material exposed under the surface layer to be analyzed using laser emission spectrometry is irradiated with one or more second laser pulses, through which the material to be analyzed is converted into the plasma state, characterized in that the one or more first and one or more several second laser pulses are generated with a single pulsed laser by multiple Q-switching during a pump cycle of the pulsed laser predetermined by a repetition rate of the laser, the first laser pulses being set with a longer laser pulse duration than the second laser pulses.

Description

Technical application area

The present invention relates to a method and a device for the preparatory laser material removal of solids, in particular for carrying out laser emission spectrometry (LIBS: Laser-Induced Breakdown Spectroscopy), wherein a surface of an object is irradiated with one or more first laser pulses to form a surface layer to remove, and material exposed under the surface layer is irradiated with one or more second laser pulses, through which the material is converted into the plasma state.

Such a method can be combined with an immediately subsequent quantitative and/or qualitative multi-element analysis of the solid material exposed beneath the surface layer using emission spectrometry. The main area of application is material analysis, as used, for example, in the sorting of metals, ores and minerals as well as in the identification of semi-finished products and products.

State of the art

Laser emission spectrometry is a non-contact analysis method for the qualitative and quantitative determination of the elemental composition of a substance, in which material is transferred from the object surface to the plasma state by focusing a pulsed laser beam on a solid object and the resulting element-specific emissions are detected using a spectrometer . For solids, the analysis result is significantly influenced by possible surface layers, through which the LIBS measurement result does not represent the sample composition of the base material to be determined. These layers can be contamination such as dirt, paint, varnish and/or oxide, corrosion and functional layers. In general, the average thicknesses of these layers depend on both the solid and the measurement location.

To generate laser-induced plasmas, irradiances of at least the order of magnitude 10 ⁹ W/cm ² are generally required. Due to the comparatively low acquisition and operating costs, flash lamp or diode-pumped solid-state lasers with a fixed repetition rate v _Rep are typically preferred for laser emission spectrometry, with the typical average optical powers of the emitted laser radiation being between 1 W and 100 W. The high irradiance levels required for laser emission spectrometry are generated by electro-optical Q-switching (O-switch) of the laser resonators, so that during a period T = 1/v _Rep , which is specified by a laser pump cycle, a pulse with a half-width (FWHM: Full Width Half Maximum) of typically between τ _L = 1 ns and τ _L = 100 ns. The duty cycle δ of the laser radiation, which in the case of rectangular modulated pulses represents the ratio between the temporal length of the pulse peak and the period length T, is in Q-switched solid-state lasers with repetition rates between v _Rep = 10 Hz and v _Rep = 100 kHz at about 10 ^-7 < δ < 10 ^-3 . Laser pulses with high-frequency Q-switched solid-state lasers with repetition rates of 1 kHz and above have pulse energies of up to E = 20 mJ and are preferably used in LIBS applications with working distances d < 10 cm between the focusing lens and the object to be analyzed. Flashlamp pumped Nd:YAG solid-state lasers with repetition rates v _Rep < 100 Hz, on the other hand, can emit laser pulses with energies of up to 1.5 J and peak pulse powers of P > 1 MW per pump cycle and are preferred for use at large working distances of d > 1 m.

During a period T, also referred to as a pump cycle in the present patent application, the Q-switching of a laser resonator can also be activated two, three or more times (n = 2.3...), so that more than one 0-switch laser pulse leaves the oscillator within a pump cycle. The totality of the Q-switched laser pulses n, which leave the laser resonator during the period T, is referred to as a laser pulse group (laser burst). Each laser pulse within a laser burst can have an individual pulse duration τ _L,j , where j is the index of the jth pulse of the laser pulse group. The time interval between two consecutive laser pulses j and j+1 in the pulse group is denoted by Δt _j . The sum of these interpulse distances between the Q-switched laser pulses Δt ₁ + Δt ₂ + ... + Δt _ni is always smaller than the period T. Laser pulses from two or more synchronously clocked oscillators or lasers can also be spatially superimposed collinearly to form a laser pulse group.

The use of a laser pulse group consisting of two collinearly propagating Q-switch pulses from a laser oscillator has several advantages. Compared to a Q-switched single pulse of energy E during a period T, a double pulse with the same total energy increases both the material removal and the laser-induced emission. In addition, the use of laser pulse groups can During a pump cycle, the analytical performance of laser emission spectrometry can also be improved.

The LIBS method enables surface analysis of a solid. The qualitative and quantitative determination of a sample composition is influenced by surface layers or contamination, the thickness of which can exceed a few 100 pm. If the surface is excessively contaminated or dirty, the measurement result obtained spectrometrically cannot represent the overall composition of the base material. To solve this fundamental problem, when using Q-switched lasers, the number N of period durations used for the LIBS measurement, in which a laser pulse group consisting of n laser pulses leaves the laser resonator, can be divided into cleaning pulse groups of the number N _pp and analysis pulse groups of the number N _mp are used, with only the latter being used for analysis evaluation. The measurement data from the cleaning or pre-pulse groups are usually discarded. Depending on the application, up to several hundred to 1000 pre-pulses are used to penetrate disturbing surface layers.

Such a method reveals, for example, the DE 103 61 727 B3 . In the method described there, the surface layer of a stationary object is at least partially removed in a first phase with a pulsed laser beam of diameter 2w ₁ and the exposed solid material is then removed in a second phase with a laser beam of a smaller diameter 2w ₂ <2w ₁ by means of laser Emission spectrometry analyzed. The change in the laser beam diameter is successively regulated by a change in the distance Δs of the point of penetration of the laser beam axis with the object surface from the focusing lens. Q-switched Nd:YAG lasers with pulse durations in the nanosecond range are used for the process. A decision criterion is presented which specifies the number of laser bursts to be applied to a sample location required for an analysis. In a specific example, the number of cleaning pulse groups to be used for a selected surface layer exceeds the value N _pp = 1000.

However, a problem with this high number of laser pulses for the analysis of a measuring point arises when the objects to be analyzed move so quickly relative to the focal position of the laser beam that they are in the measuring range of the laser beam for a time that is too short compared to the duration of the laser bursts stop. To solve such a problem, the DE 44 26 475 C2 a method for examining dirty or coated parts that are moved past the measuring system via a transport device. The device used there contains an optical guidance system, which moves the pulsed laser beam of a solid-state laser synchronously with a preselected measuring point on the moving object in such a way that successive laser pulses or laser bursts from a freely triggerable excimer laser or from a fixed-frequency Nd:YAG solid-state laser are generated several times applied one after the other to the same sample site.

The EP 1 416 265 A1 describes a scanning and a metal sorting device that detects stochastically positioned metal scrap particles with irregular surface geometry moving on a conveyor belt and carries out a LIBS measurement on these objects in order to enable subsequent sorting into different metal classes. The proposed approach is based on the use of Nd:YAG solid-state lasers, which generate a double pulse consisting of two individual laser pulses with an interpulse distance of 50 ps during a period by performing the Q-switching twice during a flash lamp cycle. In one embodiment, two laser beams from two different lasers are spatially collinearly superimposed and synchronized in time in such a way that a laser burst is obtained from four individual laser pulses. With an interpulse distance of 50 ps, the time difference between the emission of the first and last pulse in the laser burst is 150 ps. Assuming a conveyor belt speed of approximately 1.5 m/s, an object to be analyzed moves approximately 230 μm. In this publication, this offset is classified as negligible due to the laser beam diameter and there is no tracking of the laser beam during the entire interaction time between the laser beam and the object. However, this requires a laser beam diameter of at least 2.3 mm.

The superimposition of several Q-switched double-pulse lasers without movement-synchronous tracking of the pulsed laser radiation has further disadvantages in addition to the problems described above with moving objects. A corresponding LIBS system with spatial overlay of several laser systems is not economical for many industrial applications due to the multiple acquisition and operating costs. Ensuring the spatial overlay and the corresponding long-term stability of the laser bursts from several Q-switched lasers places high technical demands on the laser beam source and the overlay optics.

For a number of industrial applications, such as steel or aluminum scrap sorting, transport speeds of 3 m/s and more are specified to achieve economical throughput rates. For such high relative speeds between a measuring position and the focal position of the pulsed laser beam, a lateral offset of the interaction zones of successive laser pulses with the solid-state material is no longer negligible. This is particularly true when significantly smaller laser beam diameters than 2 to 3 mm are used and the resulting reduced degree of spatial overlap of the respective interaction zones negates the advantages of a laser burst in terms of better material removal and higher plasma emission, so that the use of laser pulse groups is no longer practical.

However, tracking the laser beam with the movement of the object requires a corresponding tracking device and suitable measurement technology, which again makes the use of such a system more expensive.

The object of the present invention is to provide a method and a device for preparatory laser material removal, which are suitable for carrying out laser emission spectrometry on moving objects and can be implemented more cost-effectively and/or enable increased analysis accuracy.

Presentation of the invention

The task is solved with the method and the device according to claims 1 and 7. Advantageous embodiments of the method and the device are the subject of the subclaims or can be found in the following description and the exemplary embodiment.

In the proposed method for preparatory laser material removal, a surface of the object to be analyzed is irradiated with one or more first laser pulses in order to remove a surface layer that interferes with the analysis. Material exposed beneath the surface layer is then irradiated with one or more second laser pulses, through which the material is converted into the plasma state in order to be able to carry out laser emission spectrometry, for example. In the method, the one or more first and second laser pulses are generated with the same pulsed laser by multiple Q-switching during a pump cycle of this laser, with the laser pulse durations of the first laser pulses being set longer than the laser pulse durations of the second laser pulses.

The method therefore uses a laser system with only one laser resonator. Within the period T specified by the repetition rate of the laser, which corresponds to the pump cycle, Q-switched laser pulses, laser pulse groups or laser bursts are emitted from this laser resonator. The total emission during a pump cycle is made up of two parts, a part with one or more first laser pulses, hereinafter also referred to as the cleaning part, and a subsequent part with one or more second laser pulses, hereinafter also referred to as the analysis part. These two parts, the cleaning part and the analysis part, are generated by suitable quality modulation with a quality modulator in the laser resonator. The pulse durations of the generated laser pulses can be adjusted via the steepness of the edge of the Q-switching so that the first laser pulses have longer laser pulse durations than the second laser pulses. Suitable quality modulators, in particular electro-optical modulators, and their control are known to those skilled in the art.

In the proposed method, the laser emission during a pump cycle and the radiation that hits the object are composed of two parts, the cleaning part and the subsequent analysis part, which is emitted from a single laser resonator by Q-switched pulse modulations during the period specified by the repetition rate of the laser become. Thus, the removal of the unwanted surface layer and the subsequent transfer of the material to be analyzed into the plasma state take place during a single pump cycle with a single laser. This enables material analyzes in which the analysis time is limited to, for example, fractions of a second due to time restrictions. In such a case, with the proposed method and the device described below, potential contamination or surface layers can be removed sufficiently quickly so that the second part of the laser emission can interact with the base material for spectroscopic analysis. The method and the device are suitable, for example, for the online sorting of metals, ores and minerals as well as the mix-up check of semi-finished products and products.

The energy and pulse duration of the one or more first laser pulses are selected so that this group of pulses is suitable as a cleaning pulse or cleaning burst for removing the surface layer. For this purpose, the first laser pulse duration is preferably selected in the microsecond range. The use of a laser pulse group enables a higher removal efficiency, measured as ablated Volume per laser burst than using just a single cleaning pulse. The second pulse group consists of one or more Q-switched laser pulses, which are preferably generated with pulse durations in the nanosecond range and with energies that allow irradiance levels of ≥ 10 ⁸ W/cm ² . These second laser pulses are therefore suitable for generating a laser-induced plasma for laser emission spectrometry. With the spatial and temporal combination of these two pulse groups in a laser burst coupled out of a resonator or a corresponding laser emission, the advantages of a laser pulse train with microsecond pulses for material removal are combined with those of a Q-switched laser burst with nanosecond pulses for emission spectrometry. This achieves faster and more efficient penetration of surface layers and thus a quicker LIBS measurement than is possible with the state of the art.

Due to the emission of both pulse groups during a pump cycle from the same resonator, superposition of two lasers is no longer necessary. This makes laser emission spectrometry more attractive and competitive for a range of applications compared to alternative methods that require multiple laser systems or complex measurement and tracking technology.

If the LIBS measurement is limited in time or space, the presented method and the associated device can also be used to analyze moving and coated objects more quickly with regard to their chemical composition. Particularly in the case of objects for which the interaction between the limitation of the measurement time, the spatial access to the measurement position and the surface coating has previously prevented a sufficiently correct analysis despite the superimposition of several Q-switched solid-state laser pulses and a movement-synchronous laser beam guidance, the accuracy of the analysis can be significantly improved by using the method described. Finally, with the method presented, depth profile analyzes can be carried out with shorter measurement times compared to conventional LIBS methods, since the laser radiation used penetrates deeper into the base material to be analyzed due to the greater material removal per unit of time. Of course, the proposed method and the associated device can also be combined with an appropriately designed beam tracking unit if necessary.

In a very advantageous embodiment, the one or more first laser pulses are generated with an average power that is smaller than the average power of the one or more second laser pulses, and a ratio of the total duration t ₁ of the cleaning part to the total duration t ₂ of the analysis part is set The following applies: t ₁ > 5*t ₂ . In addition, a total duration t ₁ of the cleaning part of >10 ps and a time interval t _A of >50 ps between the last laser pulse of the first laser pulses and the first laser pulse of the second laser pulses are advantageously set. These parameters achieve optimal results during laser ablation and subsequent analysis.

The device for carrying out the proposed method has a laser with at least one active medium and a quality modulator in a laser resonator, a pump device for optically pumping the active medium and a control device for controlling the pump device and the quality modulator. The control device controls the quality modulator in such a way that one or more first laser pulses with first laser pulse durations and then one or more second laser pulses with second laser pulse durations that are shorter than the first laser pulse durations are generated by the quality modulation during a pump cycle. The laser pulse durations and energies of the first laser pulses are chosen so that they can be used to remove a surface layer of an object irradiated with the laser pulses, while the laser pulse durations and energies of the second laser pulses are chosen so that they can transform material exposed under the surface layer into the plasma state can be transferred.

Brief description of the drawings

• 1 in schematischer Darstellung die geometrischen Verhältnisse bei der Erzeugung eines laserinduzierten Plasmas;
• 2 eine schematische Darstellung der mit dem vorliegenden Verfahren erzeugten ersten und zweiten Laserpulse bzw. Laserpulsgruppen;
• 3 ein Beispiel für die mittlere Tiefe und den mittleren Durchmesser eines mit dem Reinigungsteil der Laseremission erzeugten Kraters in Abhängigkeit von der Objektposition; und
• 4 ein Beispiel für den Aufbau der vorgeschlagenen Vorrichtung in schematischer Darstellung.

The proposed method and the associated device are explained in more detail below using an exemplary embodiment in conjunction with the drawings. Here show:

• 1 a schematic representation of the geometric conditions in the generation of a laser-induced plasma;
• 2 a schematic representation of the first and second laser pulses or laser pulse groups generated with the present method;
• 3 an example of the average depth and diameter of a crater created with the cleaning part of the laser emission depending on the object position; and
• 4 an example of the structure of the proposed device in a schematic representation.

Ways of carrying out the invention

1 shows the variables that are important when generating a laser-induced plasma on the surface of a measurement object. A focusing optics 1 focuses a pulsed laser beam 2 along the propagation axis 3 onto a surface element 4 of the measurement object. The coordinate system is chosen so that the propagation direction of the laser beam 2 runs anti-parallel to the z-axis. The normal 5 of the surface element 4 forms an angle α _L with the z-axis or the laser beam axis. The penetration point 6 of the laser beam axis through the surface element 4 corresponds to the center of the projection 7 of the laser beam cross section on the surface element 4. The distance of the focal plane 8 of the laser beam 2 from the penetration point 6 is denoted by Δs. If the focal plane 8 lies behind the penetration point 6 as viewed from the focusing optics 1, this corresponds to positive values for Δs. Typically Δs is chosen positive. The distance of the focusing optics 1 from the penetration point 6 is denoted by d.

The emission of the resulting laser-induced plasma at the puncture point 6 is collected by the receiving optics of the detector unit 9 and guided into a spectrometer. The detector unit of the spectrometer determines the time-integrated emission of the spectral lines under consideration in a defined time window. For each analyte element, the concentration of the analyte in the sample is determined in relation to the exciting laser pulse using a calibration function from the temporally integrated emissions of the spectral lines under consideration. For this purpose, it is known to calculate the input variable of the calibration function from the ratio of the emission of an analyte line under consideration to the emission of a reference line of a dominant matrix element or to a combination of the emissions of other spectral lines or a measurement signal representative of the total emission of the plasma. Laser emission spectrometry can be carried out either with a fixed or variable focal length of the focusing optics.

$${\text{t}}_1>5{\text{t}}_2,$$

$${\text{t}}_1>10\mu \text{s},$$

$${\text{t}}_{\text{A}}>50\mu \text{s}.$$

With the proposed method and the associated device, such a structure can also be used for laser emission spectrometry as part of a multi-element analysis. A laser system with only one resonator is used here. Within the period T specified by the repetition rate of the laser, Q-switched laser pulse groups or laser bursts are emitted from this resonator. The total emission consists of a cleaning part and an analysis part, each of which can consist of a series of laser pulses. This is in 2 shown schematically. Both the cleaning part 10 and the analysis part 11, which are generated within a pump cycle or the period T, can also consist of a pulse group, ie several individual pulses. The cleaning part 10 is generally described by the energy E ₁ contained in this part and the total duration t ₁ of this part. Accordingly, the analysis part 11 can be characterized by the variables E ₂ and t ₂ . In the limit case with only one laser pulse in both the cleaning portion 10 and the analysis portion 11, the sizes t ₁ and t ₂ are reduced to the respective laser pulse duration τ _L . For optimal results in laser emission spectroscopy, the following four parameters are adhered to for the laser pulse groups shown: $${\text{P}}_1<{\text{P}}_2,$$

$${\text{t}}_1>5{\text{t}}_2,$$

$${\text{t}}_1>10\mu \text{s},$$

$${\text{t}}_{\text{A}}>50\mu \text{s}.$$

The variables P ₁ = E _i /t _i and P ₂ = E ₂ /t ₂ each represent the average power of the cleaning part 10 and the analysis part 11, respectively. The average power P ₁ and the total time duration t ₁ of the cleaning part are within the The parameters listed above are specified by a parameter set (p ₁ ,...,p _w , p _w+1 ,...p _z ). The parameters p ₁ to p _w are specific to the removal process, geometry or material, while p _w+1 to p _z are laser-specific. These include, for example, the removal depth per laser pulse group, the aspect ratio of the removal depth to the crater diameter, the position of the beam waist, absorption properties of the respective material as well as the average laser power, the irradiance and the beam divergence.

3 shows a measurement example for the average depth and the average diameter of a crater created with the cleaning part 10 on the surface of a measurement object as a function of the varied object position along the propagation direction of the laser beam. To create these craters, an aluminum sample was guided perpendicular to the direction of propagation at a speed v < 0.1 m/s during the emission of a laser pulse group. During an effective interaction time of about 300 ps between the laser beam and the object, the maximum lateral displacement was 30 pm, which is negligible compared to the crater diameters determined using a light microscope. The minimum of the laser beam diameter fixes the position for which Δs = 0 applies, in this case at an object position of 16.5 cm. The maximum crater depth that can be achieved with a laser pulse group of this type is over 750 μm. With longer laser pulse groups and higher average powers, crater depths of > 1 mm can be achieved in aluminum. There In contrast, the crater depths that can be generated with a single Q-switch laser pulse with a pulse duration of T _L = 10 ns are usually well below 50 μm.

A further advantage of the method is that the crater created by the ablation process in the first partial burst, i.e. with the cleaning part, offers a larger potential interaction surface for analysis with the second partial burst, i.e. the analysis part, in the overall laser pulse group. During the first pulse group, not only is the base material located beneath the surface layer exposed, but material is also transported laterally from the created crater volume. By creating such a crater ring, the interaction surface with more representative solid material can be increased. Finally, the LIBS signals of the analysis part can be positively influenced and amplified by the generated crater geometry.

4 finally shows a schematic representation of an example of the proposed device for carrying out the method, as it can be used for combined sample preparation and LIBS analysis. A time-modulable Q-switch 24 is integrated in a laser resonator consisting of two resonator mirrors 21, 22 and an active medium 23. The laser radiation can leave the laser resonator along the optical axis 25 in one direction when the Q-switching is activated by electronics in the laser power supply 26. The laser system is controlled and parameterized by a control PC 27, which in turn is connected to peripheral devices of a measurement setup 28 for synchronization. The special feature of this device is that not only conventional Q-switch laser pulses (single, double, triple pulses, etc.) are emitted from the laser resonator during a period T via the control PC 27 and a time-modulable electro-optical Q-switch 24 but now an additional cleaning pulse or a cleaning pulse group is coupled out of the same resonator, which is generated before the Q-switched laser pulses for LIBS analysis during the same period.

The proposed method and the proposed device allow emission spectral analysis with laser-induced plasmas on stationary and moving measurement objects. Due to the ability to combine ablation and analysis laser pulse groups within a laser burst, the advantages of laser pulse trains are combined with those of a Q-switched O-switch burst with pulse durations of typically 10 ns. This approach makes it possible to penetrate solid bodies that are covered with a surface layer more quickly and efficiently with a laser beam for spectral analysis of the base material and then analyze them. The method previously used according to the state of the art of superimposing several Q-switched laser pulses for surface cleaning and subsequent analysis is improved in particular in that all of the laser radiation necessary for material processing and analysis is emitted from a single laser resonator. In addition to technological advantages, this advantage is primarily reflected in lower acquisition and operating costs. Furthermore, the described method can make greater use of the potential of non-contact multi-element laser emission spectrometry, making this analysis method more competitive with regard to industrial applications. Due to more efficient sample preparation for stationary and moving measurement objects and better penetration of contamination layers, a large amount of time is saved for laser spectroscopic analysis. With regard to sorting applications based on laser emission spectrometry, in combination with an optional laser beam guidance unit (scanner), this means higher identification rates at the same measurement rate and thus higher throughputs. Further advantages in terms of the performance of LIBS analysis arise due to the potential increase in the interaction area between the laser beam and the measurement object as well as the amplification of the laser-induced plasma due to multiple reflections and the higher absorption of the laser radiation due to the higher angle of incidence between the propagation direction and the local surface inclination.

Reference symbol list

1

Focusing optics

2

laser beam

3

Propagation axis

4

Surface element

5

Normal of the surface element

6

Penetration point

7

projection

8th

focal plane

9

Detector unit

10

Cleaning part

11

Analysis part

21

left resonator mirror

22

right resonator mirror

23

active medium

24

Q-switching

25

optical axis

26

Laser power supply

27

Control PC

28

Measurement setup

Claims (11)

Method for preparatory laser material removal for carrying out laser emission spectrometry, in which - a surface of an object is irradiated with one or more first laser pulses in order to remove a surface layer, and - material exposed under the surface layer to be analyzed by means of laser emission spectrometry with a or a plurality of second laser pulses, through which the material to be analyzed is converted into the plasma state, characterized in that the one or more first and the one or more second laser pulses are irradiated with a single pulsed laser by multiple Q-switching during a repetition rate of the laser predetermined pump cycle of the pulsed laser are generated, the first laser pulses being set with a longer laser pulse duration than the second laser pulses. Procedure according to Claim 1 , in which the one or more first laser pulses are generated with an average power that is smaller than an average power of the one or more second laser pulses, and in which for the total duration t ₁ of an emission of the first laser pulses and for the total duration t ₂ an emission of the second laser pulses applies: t ₁ > 5t2. Procedure according to Claim 2 , in which the total duration t ₁ of the emission of the first laser pulses during a pump cycle is > 10 ps and a time interval of > 50 ps is maintained between a last of the one or more first laser pulses and a first of the one or more second laser pulses. Procedure according to one of the Claims 1 until 3 , in which the second laser pulses are generated with a pulse energy that enables the material exposed under the surface layer to be irradiated with an irradiance of ≥ 10 ⁸ W/cm ² . Procedure according to one of the Claims 1 until 4 , in which a solid-state laser pumped through flash lamps or diodes is used as a pulsed laser. Procedure according to one of the Claims 1 until 5 , in which an optical emission of the material converted into the plasma state is recorded and analyzed spectroscopically. Device for preparatory laser material removal for carrying out laser emission spectrometry, with - a laser which comprises at least one active medium (23) and an electro-optical quality modulator (24) in a laser resonator, - a pump device for optically pumping the active medium (23), and - a control device (27) for controlling the pump device and the electro-optical quality modulator (24), characterized in that the control device (27) controls the electro-optical quality modulator (24) in such a way that the quality modulation during a pump cycle predetermined by a repetition rate of the laser or several first laser pulses with first laser pulse durations for removing a surface layer of an object and then one or more second laser pulses with second laser pulse durations for converting material exposed under the surface layer and to be analyzed by means of laser emission spectrometry into the plasma state, which are shorter than that first laser pulse durations are. Device according to Claim 7 , characterized in that the control device (27) controls the electro-optical quality modulator (24) and the pump device in such a way that the one or more first laser pulses are generated with an average power that is smaller than an average power of the one or more second laser pulses is, and for the total duration t ₁ of an emission of the first laser pulses and for the total duration t ₂ of an emission of the second laser pulses the following applies: t ₁ > 5t ₂ . Device according to Claim 8 , characterized in that the control device (27) controls the electro-optical quality modulator (24) in such a way that the total duration t ₁ of the emission of the first laser pulses during a pump cycle is > 10 ps and a time interval of > 50 ps between a last one or of the several first laser pulses and a first of the one or more second laser pulses is maintained. Device according to one of the Claims 7 until 9 , characterized in that the second laser pulses can be generated with a pulse energy that enables irradiation of the material exposed under the surface layer with an irradiance of ≥ 10 ⁸ W/cm ² . Device according to one of the Claims 7 until 10 , characterized in that the pump Direction is formed by one or more flash lamps or one or more diode (s).

Images