US3476968A - Microwave ion source - Google Patents

Description

Nqv. 4, 1969 L I mRo OMURA 3,476,968 S MICROWAVE ION SOURCE Filed Dec. l2, 1967 62% -He SUPPLY 20 my SAMPL L 4 El/ACU470R SUPPLY M/CROM/E El/Acmra? \7 SOURCE 3/ FIG. 2 7

SAMPLE 2 GAS SUPPLY M/CROWAI/E SOURCE INVENTOR IfZ'Ao 0/71/24 BY a @M' ATTORNEKS United States Patent U.S. Cl. 313-63 3 Claims ABSTRACT OF THE DISCLOSURE A microwave ion source in which high frequency electrodeless discharge is caused in a discharge vessel including a discharge gas by providing a microwave power in the discharge vessel to generate a discharge plasma, and the atoms or molecules of a sample gas introduced into the discharge vessel are ionized by both the photoionization effect caused by irradiating the sample gas with the resonance radiation emitted by the plasma and the charge transfer effect arising when the atoms of the discharge gas excited to a high energy in the plasma collide with the atoms or molecules of the sample gas.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a microwave ion source which efliciently ionizes a sample gas of arbitrary composition to provide a high density ion beam of the sample by utilizing high frequency discharge plasma of the gas.

Description of the prior art Recently, as a far ultraviolet light source for a vacuum monochromator a high frequency gas discharge plasma generator called a microwave discharge plasma light source has become to be employed. This plasma generator is such that a discharge gas (rare gas) such as helium gas at a pressure of, for example, the order of 0.1- mm. Hg is introduced into an intense high frequency electromagnetic field of the order of several g Hz./s. to generate a gas discharge plasma and to excite the atoms of the gas, and when the excited atoms return to their original ground state, that is, when they perform a resonance transition, they emit light of resonance line. When helium gas is employed as a discharge gas as stated above, an intense resonance radiation of a Wavelength of 584 A. is obtained.

As is well known, a resonance radiation in a far ultraviolet region emitted by such a discharge plasma has an action to ionize the atoms or molecules of a sample gas or vapour to be ionized when the atoms or molecules are irradiated with the resonance radiation, or the socalled photoionization effect. However, the previously proposed ion source employing the photoionization effect has usually little probability of ionizing the atoms or molecules of a sample gas because the ionization is caused solely by the photoionization effect, and hence the prior art device employing only the photoionization effect cannot produce a sufficient amount of ions.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a microwave ion source capable of supplying a sufficient amount of ions.

The microwave ion source of the present invention concurrently employs the aforementioned photoionization effect and the so-called charge transfer effect to improve the efficiency of ionizing a sample gas. The charge transfer effect is such a phenomenon that the atoms of a dis- 3,476,968 Patented Nov. 4, 1969 charge gas excited to a high energy state in the discharge plasma ionize a sample gas by impacting the molecules orvatoms of the sample gas, and, at the same time, the atoms of the discharge gas return to their ground state. In the ion source of the present invention the atoms of the discharge gas and the ions of the sample gas in the plasma are separated to supply only the ions of the sample" gas.

The present invention will now be described in more detail with reference to the accompanying drawings.

1 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram in longitudinal crosssection of a plasma ion source of the present invention; and

FIG. 2 is a similar diagram of another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, referring to FIG. 1, surrounding a discharge tubular vessel 1 of a good heat-proof and electrically insulating material such as a quartz tube there is provided a hollow tubular cavity resonator 2 to which a microwave source 3 is connected. One end of the discharge vessel, 1 is provided with a discharge gas inlet 4 through which a discharge gas is introduced from a discharge gas supply 8, and the other open end of the discharge vessel communicates with a vacuum chamber 5. The insides of the vacuum chamber 5 and the discharge vessel 1 communicating therewith are evacuated by evacuators 6 and 7. At the side wall of the discharge vessel 1 is provided an inlet 9 through which a sample to be ionized is introduced from a sample gas supply 10.

After the insides of the vessel 1 and the vacuum chamber 5 have been evacuated, a discharge gas, for example helium gas, is introduced from the discharge gas supply 8 into the discharge vessel 1 through the inlet 4. An appropriate pressure of the helium gas in the vessel 1 is of the order of 0.1 to 5 mm. Hg. Then, by supplying a microwave power of the order of several hundred watts at a frequency of several gHz/s. from the microwave supply 3 to the discharge vessel 1 through the resonator 2 an intense high frequency electromagnetic field is developed in the vessel 1, whereby helium atoms in the vessel 1 absorb the energy of the electromagnetic wave to be excited with a result that a gas discharge plasma 11 of helium is generated. When the helium atoms excited in the plasma 11 (hereinafter expressed by He again return to the original helium atoms at the ground state (hereinafter expressed by He), that is, when the excited helium atoms (He*) perform a resonance transition, an intense resonance radiation (hv) of a Wavelength of 584 A. is emitted. This resonance radiation (hu) of helium of 584 A. is far ultraviolet light having a high energy corresponding to a resonance potential (an energy difference between the levels before and after a resonance transition) of 21.21 ev., which has an ability to ionize the atoms of a gas the ionizing potential of which is lower than the said resonance potential 21.21 ev. Consequently, if a sample (in a gas or vapor phase) to be ionized is introduced through the sample inlet 9 into the discharge vessel 1, the atoms (or molecules) (M) of the sample in a sample space 12 are ionized by being irradiated with the resonance radiation (hi!) emitted by the plasma 11. An appropriate pressure to feed the sample gas is such that the partial pressure of the sample gas in the vessel 1 is approximately 10* to 10* mm. Hg.

The excited atoms of helium (He*) in the plasma 11 directly impact the atoms (or molecules) (M) of the sample gas in the sample space 12, giving the energy thereof to the atoms of the sample gas to ionize the sample atoms, and return to the original helium atoms (He) in the ground state. The reaction He*+M He+M+, where M+ represents the ion of the molecule (or atom) of the sample gas, is known as an ionization mechanism due to a charge transfer effect. In the present invention the ionization mechanism due to the charge transfer effect plays an important role. Therefore, it is a necessary condition of the present invention to introduce the molecules (or atoms) (M) of the sample gas to be ionized into or in the vicinity of the plasma 11 so that the molecules (M) directly collide with the excited helium atoms (He*). This is the reason why in the present invention the sample gas is introduced into the discharge vessel 1 within which the plasma 11 is generated. By this arrangement the sample gas molecules (M) are irradiated with the resonance radiation (hu) (resonance line of helium of a wavelength of 584 A.), and, at the same time, directly impacted by the excited helium atoms (He*), and hence the sample gas is efficiently ionized by undergoing both of the photo-ionization effect due to the irradiation with the resonance radiation (hp) and the ionization effect due to the charge transfer effect by the impact of the excited helium atoms (He*).

The sample gas (M+) ionized as stated above is led to behind accelerating electrodes 13, 14, and 15 by ion accelerating means consisting of the electrodes 13, 14, and 15 and a power supply 16 for providing an appropriate accelerating potential to the electrodes to form an ion beam 17, that is, the ions (M+) of the sample generated in the sample ionization space 12 near the electrode 13 are accelerated by an accelerating electric field developed between the electrodes 13, 14, and 15 and led to behind the electrodes through the central apertures of the electrodes. In this case, since the ionization efficiency in the sample space 12 is markedly enhanced as compared with the case in which only the photoionization effect is employed, the ion beam 17 is of a high density and high current. Since helium atoms (He) in the ground state, excited helium atoms (He*), and the un-ionized component (M) of the sample gas have no charge, they undergo no action of tthe ion accelerating electric field, and hence they are evacuated through outlets 1-8, 19, and 20. Thus, separation of the ions (M+) of the sample gas and the atoms of the discharge gas (helium in this case) can be attained.

The pressure in the discharge vessel 1 is of a relatively high pressure of the order of from 0.1 to 5 mm. Hg, while an ion beam transit space in the vacuum chamber 5 is usually maintained at a low pressure (high vacuum) of the order of from 1() to 10* mm. Hg. Accordingly, it is desirable to differentially evacuate the interior of the discharge vessel 1 and the ion beam transit space (right side of the electrode on the drawing) in the vacuum chamber 5 by the separate evacuators 7 and 6, respectively.

In another embodiment shown in FIG. 2, a sample gas is introduced into the vessel 1 through the inlet 4 together with helium gas. Accordingly, the sample gas is directly introduced into the plasma 11, where the sample gas is subjected to the combined ionization effect due to the photoionization effect and the charge transfer effect to be efiiciently ionized. In this embodiment a repeller electrode 21 for repelling the ions of the sample gas generated in the discharge vessel 1 towards the right open end of the vessel 1 is provided to which a repelling voltage is applied by a DC. voltage source 22. The ions (M+) of the sample gas generated in the plasma 11 are repelled towards the right open edge portion of the discharge vessel 1 by the repelling electric field developed by the repeller electrode 21, and then accelerating electric field developed between the accelerating electrodes 13, 14, and 15 and projected to the right through the central apertures of the electrodes 14 and 15 to form the ion beam 17 of high current.

The abovementioned method of introducing the sample gas into the vessel 1 together with the helium gas for generating and sustaining the plasma is advantageously applicable to an ion source for a mass spectrometer particularly when the mass spectrometer is combined with a gas chromatograph. Since a sample gas is in a mixed state with helium gas as a carrier gas in the gas chromatograph, it has been necessary, when analyzing the sample gas by means of the mass spectrometer, to separate the sample gas from the helium gas as a carrier gas before subjecting the sample gas to the mass spectrometry in order to ionize only the sample gas. However, when the ion source of the present invention is employed, the separation of the sample gas from the carrier gas is unnecessary. Because, in the ion source of the present invention, while helium gas, the ionization potential of which is of such a high value as 24.58 ev., is evacuated by the evacuators 6 and 7 without ionized in the plasma, the sample gas having such a low ionization potential as 15 ev. or lower is efiiciently ionized to be derived as an ion beam. Thus, the ion source of the present invention has in itself a function of separating the sample gas from helium gas. Consequently, the employment of the ion source of the present invention as an ion source for the mass spectrometer has the advantage that the spectrometer can be employed directly coupled with the gas chromatograph.

Generally, if it is assumed that the partial pressures of the samples are equal, the probability that the sample is photoionized by the resonance line of helium of a wavelength of 584 A. is about one hundredth the probability that the sample is ionized by electron impact. However, in the ion source of the present invention, since the partial pressure of the sample to be subjected to the photoionization can be set at about 10- to 10- mm. Hg, that is, to 1000 times larger than that in the electron impact type (usually of the order of 10- mm. Hg), the probability of photoionization is raised the more. Thus, the ion source of the present invention can provide an ion beam current of the same order as or higher than that provided by the conventional electron impact type ion source even by the photoionization effect alone. In the present invention, in addition to the photoionization effect, the ionization effect due to the charge transfer effect H +M He{-M+ caused by the collision of helium atoms (He*) excited in the plasma with the molecules or atoms (M) of a sample gas introduced into or around the plasma contributes to the ionization, and hence the ion current increases the more. The degree of the ionization due to the excited atoms (He*) becomes higher as the pressure of helium gas introduced into the plasma gets higher and the microwave power provided to the plasma gets larger. Therefore, the derived ion beam current can be varied by adjusting the pressure of the helium gas introducd into the discharge vessel and the supplied microwave power.

An experiment has shown that the ion current provided by the ion source concurrently employing both the ionization reactions due to the photo-irradiation and the charge transfer effect of the present invention can be made 10 times larger than that obtained from the conventional electron impact type or photoionization type ion source.

Further, since the pressure within the plasma in the ion source of the present invention is comparatively high, ie 0.1 to 5 mm. Hg, the density of gas atoms is high. Consequently, the temperature of the generated ions is comparatively low, and hence no thermal decomposition of the molecules of the sample takes place. Moreover, the width of the energy distribution of the generated ions is very narrow.

I claim:

1. A microwave ion source comprising:

a vacuum chamber;

evacuating means for evacuating the interior of said vacuum chamber;

a discharge vessel communicating with said vacuum chamber; a

means for introducing a discharge gas into said discharge vessel;

means for providing a high frequency power in said discharge vessel to generate a high frequency gas discharge plasma;

means for introducing a sample gas to be ionized into said discharge vessel so that said sample gas is ionized both by being irradiated with the resonance radiation emitted by excited atoms of said discharge gas in said plasma at their resonance transition and by being impacted by said excited atoms; and

ion accelerating means for deriving only ions of said sample gas generated in said discharge vessel into said vacuum chamber.

2. Microwave ion source according to claim 1, wherein said discharge gas is helium gas.

3,254,209 5/1966 Fite et a1. 2504l.9

JAMES W. LAWRENCE, Primary Examiner 0 RAYMOND F. HOSSFELD, Assistant Examiner U.S. Cl. X.R.

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