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Resonance ionisation of Ga atoms
Volume
106, number
CHEMICAL
3
PHYSICS
20 April 1984
LETTERS
RESONANCE IONKATION OF Ga ATOMS R. WUNDERLICH Max-Hawk-lnsitut
Received
fir
18 January
and T. KIRSTEN Kcmphysik.
P-0. Box
103 980.
6900
Ileidelberg,
Federal
Republic
of Gcmraqv
1984
Saturation of the photoionintion of Ca atoms by resonance ionisation is demonstrated. The photoionisation cross scction of the excited 4s2( lS),, state is measured to be 3 X 1O-‘8 cmz_ Saluration (100% ionintion) of a given sample is achieved at power levels of the order of 100 hfM’/cmZ.
ionisation
1. Introduction
2. Resonance
Resonance ionisation is a multiphoton ionisation process including one or more discrete absorption steps. The discrete absorption causes chemically selective ionisation. With sufficiently high laser intensities il is possible to saturate ionisation. Then a whole ground-state population is converted into ions and electrons. The method was introduced by Hurst (1977) and demonstrated with the detection of single Cs atoms in a proportional counter [ I]. Single-atom detection is particularly desirable in experiments dealing with rare events such as the radiochemical Ca solar neutrino experiments [2] or double beta decay experiments 131. The applicability of this method to a Li solar neutrino experiment has been investigated by Kramer et al. [4] and Letunann et al
First, we have investigated the resonance ionisation of Ga in an atomic-beam experiment in order IO establish the conditions for the saturation of this process. The experimental conditions chosen for the one-photon resonant, two-photon ionisation of Ga are based on the general treatment of such processes by Hurst et al. (61. Generally, when a broad-band laser is used for the excitation of the discrete resonance = ;aVD, AuL being the laser bandwidth and Au, @L the Doppler bandwidth of the atomic absorption), every atom in an inhomogeneously broadened gas takes part in the absorption process. Furthermore, if
PI In radiochemical neutrino experiments, the nuclei produced by neutrino capture (inverse beta decay) serve to monitor the incident neutrino fluxes. The product nuclides are detected by their radioactive decay. Their absolute number is always very small; therefore, background reduction in the counting device is of utmost importance. For instance, only about 20 germanium-7 1 atoms per exposure cycle are produced in the Ca solar neutrino experiment [2]. Single-atom detection of the daughter atoms (Ca) following the decay of their parents (Ge) can provide an unambiguous signature of genuine decays as op posed to background events. This is why we investigate resonance ionisation of Ga. 242
of Ga
the excitation is sufficiently intense, the ground-state and excited-state populations are levelled over the whole absorption profile. Using known transition data [7], we have calculated the required intensity for population levelling to be 4 X i04 W/cm2 for the 4s2( 1S)4d3D resimance state. The conditions for the flux and fluence of the ionisation laser which must be met to achieve saturation of the ionisation were given by Hurst [ 1] _ Much work has been done in investigations of the applicability of rate equations to these processes [6, 8,9]. Under the conditions of our experiment (atomic beam, laser bandwidth of IO CHz and an interaction time of 20 ns), rate equations can be applied to yield quantitative estimates of the laser intensities which are necessary to achieve saturation for different values of the photoionisation cross section o,_
We calculated
energy In
20 xoril 198-l
CHEMICAL PHYSICS LETTERS
Volume 106. number 3
the ionisation
yield
in order
to develop
the conditions for saturation of Ga ionisarion. The value of u, required for this purpose is expected to lie in the range from lo-l8 to IO-t7 cm2. This is a value typical of atoms with this electronic configuration [ 1 I 1. Let Ft be the intensity of the laser for
“-1
40000
the discrete transition and Fz the intensity of the ionisation laser. A calculation of the ioniation probability after a 20 ns laser (square) pulse versus F2 WIS carried out for F, fixed but high enough to ensuresaturation of the discrete transition. Accordingly.saturation is approached typically at F2 in the order of some 1O36 photons/cm3 s (some 20 hlW/cm3 j.
35000
30000
3. Experimental 25000 24000
The experimental
1000 C Fig. l_ Part of the gallium level scheme ionization pathway indicated.
Fig. I shows
some
of the lower
with the resonance
energy
levels of the
gallium atom with the pathway of the chosen resonant ionisation process indicated. The 4sz( 1 S)4d3,3 resonance state is preferred mainly because this state can ionised by photons of a ruby laser (1.78 eV) capable of providing high intensities with beam width up to 10 mm, such that large volumes can be swept. This is important in the intended applications as mentioned
apparatus
is sketched
in figs. 2
and 3. The output of a ruby Iaser is frequency-doubled to pump a dye laser (dye R 6G) whose output wavelength of 574 nm is again frequency-doubled to gencrate the 787.32 nm radiation for the escitation of the discrete resonance. The ground wave of the ruby laser at 694 run and the harmonic output are separated with a dichroic beam splitter_ The pulse energies are 800 and 180 mJ, respectively. The pulse shape of the
ground wave is nearly gaussian with a halfwidth of 30 ns. The output of the frequency-doubled dye laser has a pulse ener,rg of 3 mJ. It was verified esperimentally that the intensity of the frequency-doubled dye output is high enough to saturate the discrete transition for a time interval of at least 70 ns. The dye-laser beam was expanded with
above. The 4s*(J WP~/Z state of the ground-state electronic configuration is a mctnstable state and can be populated by spontaneous emission from the 4s’( 1S)4d3,2 resonance state, at a rate of 2.7 X IO7 s-l
[JO]. In atomic-beam lisional equilibration, sink. This should
experiments, i.e. without colthis state acts as a population
be accounted
for in a quantitative
description of the process. Spontaneous emission from the 4&l S)4d,lz state to the 4s?-( ~S)SP~~,~,~ statescan be neglected in comparisonwith the spontaneous emission to the ground state because the Einstein coefficients are proportional to Z.J~and the matrix elements are expected to have the same order of magnitude.
Fig. 2. Sketch of the optical system. FDI and FDZ, frequency doublers; Dl and DZ. dicbroic mirrors; BC. BE, beam condensation and espanding telescopes. respectively; Fl, F2, filters; N. variable stack of neutral density tilters.
Volume
106. number
CHEhIICAL_PHYSICS
3 Detection
LETTERS
20 April 1984
Circuit
Ll I
L2 Fig. 3. Detection circuit consisting of a drift chamber and electronics. The atomic beam is perpendicualr to the page inside :he drift chamber. K. cathode; KG. cathode grid; GP, guard plates; AG, anode grid; A, munting wire;S, grounded shielding; PD. pyroeiectricjoulemeter: Pl, P2, photodiodes: PA, h!A. pre- and main-amplifiers; DL, delay amplifier; LG. linear gate; MCA, multichannel analyser; DPA. optional dual parameter analyser.
the telescope BE. The output of the ruby laser at 694 run can be condensed. The beam diameter at the interaction volume was 4 mm. The UV beam and the 694 run radiation are combined at a second dichroic mirror D2 and directed into the detector. The energy of the ionisation’laser was measured after passing the dectector and a filter, RG 590, with a calibrated pyroelectric detector. The beam diameter was measured with burn paper to be 4 nun. The laser beams were apertured before the chamber to allow only the near-uniformintensity beam center of the 694 run beam to pass into the detector. In the present experiment, an atomic beam of Ga is directed into a drift chamber and crossed with the two laser beams. Under these conditions, the sensitivity is limited by the noise of the charge-sensitive preamplifier to 3 X lo3 elementary charges. For later single-atom detection modes, gas amplification will be required_ The drift chamber is made of quartz plates carefully aligned to avoid back reflections from the windows to the platinated nickel electrodes. The drift 244
region is formed by the massive cathode K and the anode grid AC. A counting wire A allows the chamber to be used in the gas amplification mode. The cathode is shielded by the cathode grid KC to capture photoelectrons originating from the cathode. Two field plates beneath the anode grid are held at the same potential to allow charges to be collected only from a defmite volume. Ail electrodes are held on positive potentials. The drift chamber is connected to a pumping system with turbo-pump, getter pump and cold trap. When working under atomic-beam conditions, the resonantly produced electrons are collected on the anode grid held at +60 V with the counting wire grounded. The atomic beam is produced in a quartz furnace attached to the bottom of the drift chamber between the two electrodes. The divergence of the beam is about 5”. At a sample temperature of 66O”C, the gallium concentration in the detection volume is 4 -5 X lo5 atoms/cm 3. The operating pressure was X2 X 10m6 Torr. Generally, the partial pressure of 02
Volunrc 106, nunrbcr 3
CHEMICAL
PHYSICS
LETTERS
should be < tom5 Torr to avoid both surface passivation and reaction
of free Ga atoms
with
osygen.
In
20 April 198-l
1
Ga Resonance
150
1
I
I
I
Iomsahon
this respect, the getter pump is particularly useful when working with a counting gas of pure argon. The pulses from the charge-sensitive preamplifier are measured on a storage oscilloscope. In blank runs, zero signal was obtained under a wide range of potentially relevant conditions. The fluence was varied from 0.3 to 3 J/cm? (150 hlW/cm2) and the operating
pressure
from
2 X 10-6
Torr
resi-
All charge measurements were done one the collecting anode grid with a sensitivity limit of 3 X IO3 elementary charges (&IO mV signal). dual gas to 450
Torr
argon.
4. Results The ionisation signal was measured as a function of the wavelength of the UV laser while the intensity F2 of the ionisation laser was kept at a value known to saturate the ionisation step. The result is shown in fig. 4. The width of the signal is due to the laser bandwidth. Off-resonance signals were not detected. Fig. 5 shows the ionisation signal versus the intensity F2 of the ionisation laser. The intensity F, of the laser for the discrete transition was kept at a value
I:g. 5. lonisation signal versus the intensity of the ionisation laser. F2, with FI constant.
sufficient to saturate the discrete transition. The solid curve is obtained from model calculations with the absolute value of o, = 3 X IO-J8 cm?. It is worth noting that. with this kind of esperiment and data evaluation, there is no need for knowledge of the absolute Ga concentration in the interaction volume. Only F2 has to be known. It can be calculated from the energy measured per pulse and the beam diameter. Treating the laser pulse as a square pulse introduces an error on the order of 20%. Saturation behavior at a fluencc of 2.5 J/cm2 for the ionisation laser is inferred both from the esperimental data and from the calculations.
5. Discussion
With the apparatus described above. resonance isation of Ga atoms can be saturated via the @(I S)4d,,, resonance state. The photoionisation cross section is 3 X 1O- l8 cm?. Using an atomic
1000 -
beam.
no off-resonance
signal (detection
limit
ion-
3 X IO3
elementary charges) could be detected. Saturation is reached at a fhtence of 2.5 J/cm7 for the ionigtion laser. This high value (corresponding to !OO hlW/cm2) might produce off-resonance or straylight background effects in 3 single-atom-detection esperiment in a real counting
gas. In this respect,
it is advisable
two-discrete-step-ihree-photon-ionisotion
F&,. 4. Resonance ionisation signal versus excitation length with Fz constant.
v.ave-
to use a scheme with
the 4s2( I S)Ss,,, state as the !irst resonance and one of the rzp states as a second resonance. Advantages are obvious. The wavelength for the escitation of the first resonance is 403.2 nm, which makes the photo-
24.5
Volume 106;ntimber 3
CHEMICAL
electron-production compared to the 287.4 nm radiation less probable: Ttie se&d giscrete resonance re‘quires a wavelength wh;ch is even higher. The photoiW.kattin C?SS Sec~iBnof the+ St&?5 is expsr&?d tO be higher than 10-l’ cm2. Thus; more than one order of magnitude can be saved in F2, solving all problems connected with the high 100 MW/cm* powei density. Letokhov [ 121 reported a photoionisation cross sectiqn for the 4s2(1S)5s state of 4 X 10-l’ cm? Direct ionisation of this state does not seem to be advisable because there is no light source lo saturate the ioriisation step without focusing, thus limiting the active volume and giving only a poor volume efficiency in possible counter applications of &is method. With a two-step discrete ionisation process, high-lying Rydberg states can be excited. Ionisation can be completed with 100% efficiency by an electric field.
246
PHYSICS
LETTERS
.20
April 1984
References [ I] G.S. Hurst, KG. Payne and J.P. Young, Phys. Rev. A15 (1977) 2282 121 W. Hampel, in: Science Underground, AIP Conference Proceedings 96. ed. h1.N. Nieto (1983) p. 88. 131 T. Kirsten, in: Science Underground, AIP Conference Proceedings 96 ed. M.N. Nieto (1983) p. 396. 14) S.D. Kramer, J.P. Young, G.S. Hurst and M.G. Payne, opt. commlm. 30 (1979) 47. IS] B.E. Lehmann, SD. Kramer. S.L. AUman.G.S. Hurst and M.G. Payne, Chem. Phys. Letters 71 (1980) 91. [6] G.S. Hurst. M.G. Payne, SD. Kramer and 3-P. Young, Rev. Mod. Phys. 51 (1979) 767. [7] J. hligdalek. Can. J. Phys. 54 (1976) 118. [S] J.R. Aikerhalt and J.H. Eberly, Phys. Rev. Al4 (1976) 1705. [9] S. Swain, J. Phys. B12 (1979) 1213. [lo] J. htigdalek and W.E. Baylis, J. Phys. B12 (1979) 2595. (111 F. Combet Farnous, J. Phys. (Paris) 30 (1969) 521. [ 121 VS. Letokhov. V.I. Mishin, N. Eshkobilov and A.T. Tursunov. Opt. Commun. 41 (1982) 331.
106, number
CHEMICAL
3
PHYSICS
20 April 1984
LETTERS
RESONANCE IONKATION OF Ga ATOMS R. WUNDERLICH Max-Hawk-lnsitut
Received
fir
18 January
and T. KIRSTEN Kcmphysik.
P-0. Box
103 980.
6900
Ileidelberg,
Federal
Republic
of Gcmraqv
1984
Saturation of the photoionintion of Ca atoms by resonance ionisation is demonstrated. The photoionisation cross scction of the excited 4s2( lS),, state is measured to be 3 X 1O-‘8 cmz_ Saluration (100% ionintion) of a given sample is achieved at power levels of the order of 100 hfM’/cmZ.
ionisation
1. Introduction
2. Resonance
Resonance ionisation is a multiphoton ionisation process including one or more discrete absorption steps. The discrete absorption causes chemically selective ionisation. With sufficiently high laser intensities il is possible to saturate ionisation. Then a whole ground-state population is converted into ions and electrons. The method was introduced by Hurst (1977) and demonstrated with the detection of single Cs atoms in a proportional counter [ I]. Single-atom detection is particularly desirable in experiments dealing with rare events such as the radiochemical Ca solar neutrino experiments [2] or double beta decay experiments 131. The applicability of this method to a Li solar neutrino experiment has been investigated by Kramer et al. [4] and Letunann et al
First, we have investigated the resonance ionisation of Ga in an atomic-beam experiment in order IO establish the conditions for the saturation of this process. The experimental conditions chosen for the one-photon resonant, two-photon ionisation of Ga are based on the general treatment of such processes by Hurst et al. (61. Generally, when a broad-band laser is used for the excitation of the discrete resonance = ;aVD, AuL being the laser bandwidth and Au, @L the Doppler bandwidth of the atomic absorption), every atom in an inhomogeneously broadened gas takes part in the absorption process. Furthermore, if
PI In radiochemical neutrino experiments, the nuclei produced by neutrino capture (inverse beta decay) serve to monitor the incident neutrino fluxes. The product nuclides are detected by their radioactive decay. Their absolute number is always very small; therefore, background reduction in the counting device is of utmost importance. For instance, only about 20 germanium-7 1 atoms per exposure cycle are produced in the Ca solar neutrino experiment [2]. Single-atom detection of the daughter atoms (Ca) following the decay of their parents (Ge) can provide an unambiguous signature of genuine decays as op posed to background events. This is why we investigate resonance ionisation of Ga. 242
of Ga
the excitation is sufficiently intense, the ground-state and excited-state populations are levelled over the whole absorption profile. Using known transition data [7], we have calculated the required intensity for population levelling to be 4 X i04 W/cm2 for the 4s2( 1S)4d3D resimance state. The conditions for the flux and fluence of the ionisation laser which must be met to achieve saturation of the ionisation were given by Hurst [ 1] _ Much work has been done in investigations of the applicability of rate equations to these processes [6, 8,9]. Under the conditions of our experiment (atomic beam, laser bandwidth of IO CHz and an interaction time of 20 ns), rate equations can be applied to yield quantitative estimates of the laser intensities which are necessary to achieve saturation for different values of the photoionisation cross section o,_
We calculated
energy In
20 xoril 198-l
CHEMICAL PHYSICS LETTERS
Volume 106. number 3
the ionisation
yield
in order
to develop
the conditions for saturation of Ga ionisarion. The value of u, required for this purpose is expected to lie in the range from lo-l8 to IO-t7 cm2. This is a value typical of atoms with this electronic configuration [ 1 I 1. Let Ft be the intensity of the laser for
“-1
40000
the discrete transition and Fz the intensity of the ionisation laser. A calculation of the ioniation probability after a 20 ns laser (square) pulse versus F2 WIS carried out for F, fixed but high enough to ensuresaturation of the discrete transition. Accordingly.saturation is approached typically at F2 in the order of some 1O36 photons/cm3 s (some 20 hlW/cm3 j.
35000
30000
3. Experimental 25000 24000
The experimental
1000 C Fig. l_ Part of the gallium level scheme ionization pathway indicated.
Fig. I shows
some
of the lower
with the resonance
energy
levels of the
gallium atom with the pathway of the chosen resonant ionisation process indicated. The 4sz( 1 S)4d3,3 resonance state is preferred mainly because this state can ionised by photons of a ruby laser (1.78 eV) capable of providing high intensities with beam width up to 10 mm, such that large volumes can be swept. This is important in the intended applications as mentioned
apparatus
is sketched
in figs. 2
and 3. The output of a ruby Iaser is frequency-doubled to pump a dye laser (dye R 6G) whose output wavelength of 574 nm is again frequency-doubled to gencrate the 787.32 nm radiation for the escitation of the discrete resonance. The ground wave of the ruby laser at 694 run and the harmonic output are separated with a dichroic beam splitter_ The pulse energies are 800 and 180 mJ, respectively. The pulse shape of the
ground wave is nearly gaussian with a halfwidth of 30 ns. The output of the frequency-doubled dye laser has a pulse ener,rg of 3 mJ. It was verified esperimentally that the intensity of the frequency-doubled dye output is high enough to saturate the discrete transition for a time interval of at least 70 ns. The dye-laser beam was expanded with
above. The 4s*(J WP~/Z state of the ground-state electronic configuration is a mctnstable state and can be populated by spontaneous emission from the 4s’( 1S)4d3,2 resonance state, at a rate of 2.7 X IO7 s-l
[JO]. In atomic-beam lisional equilibration, sink. This should
experiments, i.e. without colthis state acts as a population
be accounted
for in a quantitative
description of the process. Spontaneous emission from the 4&l S)4d,lz state to the 4s?-( ~S)SP~~,~,~ statescan be neglected in comparisonwith the spontaneous emission to the ground state because the Einstein coefficients are proportional to Z.J~and the matrix elements are expected to have the same order of magnitude.
Fig. 2. Sketch of the optical system. FDI and FDZ, frequency doublers; Dl and DZ. dicbroic mirrors; BC. BE, beam condensation and espanding telescopes. respectively; Fl, F2, filters; N. variable stack of neutral density tilters.
Volume
106. number
CHEhIICAL_PHYSICS
3 Detection
LETTERS
20 April 1984
Circuit
Ll I
L2 Fig. 3. Detection circuit consisting of a drift chamber and electronics. The atomic beam is perpendicualr to the page inside :he drift chamber. K. cathode; KG. cathode grid; GP, guard plates; AG, anode grid; A, munting wire;S, grounded shielding; PD. pyroeiectricjoulemeter: Pl, P2, photodiodes: PA, h!A. pre- and main-amplifiers; DL, delay amplifier; LG. linear gate; MCA, multichannel analyser; DPA. optional dual parameter analyser.
the telescope BE. The output of the ruby laser at 694 run can be condensed. The beam diameter at the interaction volume was 4 mm. The UV beam and the 694 run radiation are combined at a second dichroic mirror D2 and directed into the detector. The energy of the ionisation’laser was measured after passing the dectector and a filter, RG 590, with a calibrated pyroelectric detector. The beam diameter was measured with burn paper to be 4 nun. The laser beams were apertured before the chamber to allow only the near-uniformintensity beam center of the 694 run beam to pass into the detector. In the present experiment, an atomic beam of Ga is directed into a drift chamber and crossed with the two laser beams. Under these conditions, the sensitivity is limited by the noise of the charge-sensitive preamplifier to 3 X lo3 elementary charges. For later single-atom detection modes, gas amplification will be required_ The drift chamber is made of quartz plates carefully aligned to avoid back reflections from the windows to the platinated nickel electrodes. The drift 244
region is formed by the massive cathode K and the anode grid AC. A counting wire A allows the chamber to be used in the gas amplification mode. The cathode is shielded by the cathode grid KC to capture photoelectrons originating from the cathode. Two field plates beneath the anode grid are held at the same potential to allow charges to be collected only from a defmite volume. Ail electrodes are held on positive potentials. The drift chamber is connected to a pumping system with turbo-pump, getter pump and cold trap. When working under atomic-beam conditions, the resonantly produced electrons are collected on the anode grid held at +60 V with the counting wire grounded. The atomic beam is produced in a quartz furnace attached to the bottom of the drift chamber between the two electrodes. The divergence of the beam is about 5”. At a sample temperature of 66O”C, the gallium concentration in the detection volume is 4 -5 X lo5 atoms/cm 3. The operating pressure was X2 X 10m6 Torr. Generally, the partial pressure of 02
Volunrc 106, nunrbcr 3
CHEMICAL
PHYSICS
LETTERS
should be < tom5 Torr to avoid both surface passivation and reaction
of free Ga atoms
with
osygen.
In
20 April 198-l
1
Ga Resonance
150
1
I
I
I
Iomsahon
this respect, the getter pump is particularly useful when working with a counting gas of pure argon. The pulses from the charge-sensitive preamplifier are measured on a storage oscilloscope. In blank runs, zero signal was obtained under a wide range of potentially relevant conditions. The fluence was varied from 0.3 to 3 J/cm? (150 hlW/cm2) and the operating
pressure
from
2 X 10-6
Torr
resi-
All charge measurements were done one the collecting anode grid with a sensitivity limit of 3 X IO3 elementary charges (&IO mV signal). dual gas to 450
Torr
argon.
4. Results The ionisation signal was measured as a function of the wavelength of the UV laser while the intensity F2 of the ionisation laser was kept at a value known to saturate the ionisation step. The result is shown in fig. 4. The width of the signal is due to the laser bandwidth. Off-resonance signals were not detected. Fig. 5 shows the ionisation signal versus the intensity F2 of the ionisation laser. The intensity F, of the laser for the discrete transition was kept at a value
I:g. 5. lonisation signal versus the intensity of the ionisation laser. F2, with FI constant.
sufficient to saturate the discrete transition. The solid curve is obtained from model calculations with the absolute value of o, = 3 X IO-J8 cm?. It is worth noting that. with this kind of esperiment and data evaluation, there is no need for knowledge of the absolute Ga concentration in the interaction volume. Only F2 has to be known. It can be calculated from the energy measured per pulse and the beam diameter. Treating the laser pulse as a square pulse introduces an error on the order of 20%. Saturation behavior at a fluencc of 2.5 J/cm2 for the ionisation laser is inferred both from the esperimental data and from the calculations.
5. Discussion
With the apparatus described above. resonance isation of Ga atoms can be saturated via the @(I S)4d,,, resonance state. The photoionisation cross section is 3 X 1O- l8 cm?. Using an atomic
1000 -
beam.
no off-resonance
signal (detection
limit
ion-
3 X IO3
elementary charges) could be detected. Saturation is reached at a fhtence of 2.5 J/cm7 for the ionigtion laser. This high value (corresponding to !OO hlW/cm2) might produce off-resonance or straylight background effects in 3 single-atom-detection esperiment in a real counting
gas. In this respect,
it is advisable
two-discrete-step-ihree-photon-ionisotion
F&,. 4. Resonance ionisation signal versus excitation length with Fz constant.
v.ave-
to use a scheme with
the 4s2( I S)Ss,,, state as the !irst resonance and one of the rzp states as a second resonance. Advantages are obvious. The wavelength for the escitation of the first resonance is 403.2 nm, which makes the photo-
24.5
Volume 106;ntimber 3
CHEMICAL
electron-production compared to the 287.4 nm radiation less probable: Ttie se&d giscrete resonance re‘quires a wavelength wh;ch is even higher. The photoiW.kattin C?SS Sec~iBnof the+ St&?5 is expsr&?d tO be higher than 10-l’ cm2. Thus; more than one order of magnitude can be saved in F2, solving all problems connected with the high 100 MW/cm* powei density. Letokhov [ 121 reported a photoionisation cross sectiqn for the 4s2(1S)5s state of 4 X 10-l’ cm? Direct ionisation of this state does not seem to be advisable because there is no light source lo saturate the ioriisation step without focusing, thus limiting the active volume and giving only a poor volume efficiency in possible counter applications of &is method. With a two-step discrete ionisation process, high-lying Rydberg states can be excited. Ionisation can be completed with 100% efficiency by an electric field.
246
PHYSICS
LETTERS
.20
April 1984
References [ I] G.S. Hurst, KG. Payne and J.P. Young, Phys. Rev. A15 (1977) 2282 121 W. Hampel, in: Science Underground, AIP Conference Proceedings 96. ed. h1.N. Nieto (1983) p. 88. 131 T. Kirsten, in: Science Underground, AIP Conference Proceedings 96 ed. M.N. Nieto (1983) p. 396. 14) S.D. Kramer, J.P. Young, G.S. Hurst and M.G. Payne, opt. commlm. 30 (1979) 47. IS] B.E. Lehmann, SD. Kramer. S.L. AUman.G.S. Hurst and M.G. Payne, Chem. Phys. Letters 71 (1980) 91. [6] G.S. Hurst. M.G. Payne, SD. Kramer and 3-P. Young, Rev. Mod. Phys. 51 (1979) 767. [7] J. hligdalek. Can. J. Phys. 54 (1976) 118. [S] J.R. Aikerhalt and J.H. Eberly, Phys. Rev. Al4 (1976) 1705. [9] S. Swain, J. Phys. B12 (1979) 1213. [lo] J. htigdalek and W.E. Baylis, J. Phys. B12 (1979) 2595. (111 F. Combet Farnous, J. Phys. (Paris) 30 (1969) 521. [ 121 VS. Letokhov. V.I. Mishin, N. Eshkobilov and A.T. Tursunov. Opt. Commun. 41 (1982) 331.