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High resolution spectroscopy using photodeflection
Volume
16, number
1
OPTICS COMMUNICATIONS
HIGH RESOLUTION
A.F. BERNHARDT
January
SPECTROSCOPY USING PHOTODEFLECTION
1976
*
*, D.E. DUERRE, J.R. SlMPSON and L.L. WOOD
University of Califortzia Lanlrence I.ivermore Laboratory, Livermore, CA 94550, USA Received
22 July
1975. revised
Isotope separation by used to detcrminc optical 5536 A resonance. Several solved with the technique
manuscript
received
3 September
1975
laser deflection of an atomic beam, combined with simultaneous mass spectroscopy, has been frequency shifts and to assign mass numbers to all components of the Ba 6s’ i So-6s6p ‘I’, components which cannot be resolved optically without the use of enriched samples, were rcdescribed. They are 13’Ba(F= S/2) at 120 MHz, ‘36Ba at 128 Mllz and ‘34Ba at 138 hlH7.
The laser deflection of a single isotopic component of an atomic beam has recently been demonstrated [ 11. The technique relies on the selective absorption of laser beam photons by one isotopic component of an atomic beam to transfer momentum from the laser beam to that component. In a well-collimated atomic beam, the momentum acquired by many absorption and spontaneous re-emission events per atom will be sufficient to deflect the isotopic component out of the original beam. This physical separation permits mass analysis of the deflected beam and highly reliable mass assigntnent of various isotopic hyperfine components. In addition, peaks from different isotopic components which cannot be resolved optically, can be unequivocally resolved from the associated mass spectrometer data. This is true since plot of mass peak height as a function of laser frequency gives a separate absorption curve for each mass number. The experimental set-up has been described previously 111 and is presented in fig. 1. An atomic beam of bar-ium is irradiated at right angles with light from an Ar-pumped cw dye laser. The divergence of the atomic beam along the laser axis is 8 mrad. The dye
Work pcrformcd under the auspices of the U.S. Energy Research and Development Administration. Also I:annic and John Ilertz Foundation l:ellovv at the Dt+ partment of Applied Science. University of California at Davis-Livcrmorc.
CW dye laser
iv =
Quadrupole mall
6 MHz
spectrometer
Detecmn
,I,,+
dmde
Fig. 1. Experimental
set up.
laser (Spectra Physics 270/581) t operates on a solution of 2 X 10e4 M Rhodamine 6G in a 3/l mixture of water and hexafluoro-2-propanol. Its output at the 5536 A resonance of barium is 40 mW, with a fwhm linewidth of 5 6 MHz. Before entering the vacuum ? The research equipment and materials designated by brand name in this paper are for identification purposes only, and do not imply the endorsement of the identified equipment or materials by the University of California or the U.S. Energy Research and Development Administration to the exclusion of any other equipment or materials made by other manufacturers which may be equally well suited to the purposes of the work reported.
Volume
16, number
OPTICS COMMUNICATIONS
1
I
.,a 6.6 -
.4 -
.3 -
2
-
.I -
G\
VI -60
I
I
I
0
60
120
b! 180
240
300
360
1
’
1
420
480
540
LJ
Fig. 2. Optical fluorescence spectrum of the barium 6sz ‘So-6s6p ’ P1 resonance. Frequency shifts of the various isotopic-hyperfine components are referred to the center of the ‘38Ba resonance.
system, the laser beam is expanded by a factor of 10 through a cylindrical telescope. Irradiance along the atomic beam is approximately 44 mW/cm2. Radiation scattered by the atomic beam is detected by a photomultiplier tube. A trace of PM output as a function of optical frequency is presented in fig. 2. Six fluorescence peaks are discerned. The largest peak is identified with 138Ba, the most abundant (72%) barium isotope. Shifts are referred to the center of this peak. Twenty centimeters above the base of the irradiation region, a 4 mm wide slit divides the atomic beam, passing only the deflected portion. Directly behind it lies the ionizer of a quadrupole mass spectrometer (EAI Quad 200). The atomic beam enters the ionizer perpendicular to the axis of the quadrupole unit. Barium atoms in the irradiation region absorb and reradiate on the 5536 a transition until decay to the metastable 6s5d 1D2 state occurs from the 6s6p 1P, state. Atoms in the 1 D2 state cease to absorb and cannot be further deflected. Metastable state accumulation is nearly complete. When the laser is tuned to the 138Ba resonance, the upper portion of the atomic beam (that farthest from the beam source) becomes very faint in resonantly scattered 5536 a light. The bottom portion, by contrast, is extremely bright. (This effect is not due to Doppler shift as a result of deflection, because when a second laser operating at
January
1976
5826 a is used to depopulate the metastable state via excitation to the 5d6p 1PI state, the upper portion of the beam does not diminish in intensity of scattered 553.5 a radiation.) Thus most deflected atoms enter the ionizer of the mass spectrometer in the metastable 6sSd ID2 state. The electron energy in the ionizer was set to 10 eV. Such a low energy had the advantage of eliminating hydrocarbon background. It also made the ionizer about twice as efficient in generating ions from metastable atoms than from ground state atoms. The mass spectrometer output was filtered to eliminate r.f. pickup, amplified using a Tektronix 1A7 plug-in, and averaged by means of a Princeton Applied Research TDH9 Waveform Eductor. In this manner, atom densities as low as 3 X lo4 atoms/cc could be reliably detected. A quadrupole unit of more recent design could improve this figure. The frequency standard for the experiment was a Spectra Physics model 470 Optical Spectrum Analyzer with a free spectral range of 2 GHz and resolution of 6 MHz at 5536 8. For stability, the analyzer had been temperature-stabilized and sealed hermetically to prevent longer term drift due to changes in atmospheric pressure. The instrument did not drift observably (< 1 MHz) during the course of experimentation: its “zero-setting” was checked at five to ten minute intervals against the 13*Ba resonance peak. Data was taken in the following manner: the frequency of t‘he laser was changed manually in 8 MHz steps. After averaging for 10 (or in one run, 20) seconds, a picture of an oscilloscope trace of the output of the TDH9 was taken. Four runs, covering the region between -80 and +640 MHz, were made in this way. The height of isotope peaks of mass 134 to 137 were measured and plotted against optical frequency. Electronic instability of the mass analyzer at low signal levels gave some output amplitude uncertainties. Some of this uncertainty was removed by averaging the four runs so that, except for the weak F= i component of the 135Ba and 137Ba peaks, the absolute error is 2 6 MHz. The latter peaks were too close to the sensitivity limit of the mass spectrometer to even be this accurate and the data merely confirms their presence under the peak at 530 MHz in fig. 2. Center frequencies for all peaks in the isotopic hyperfine structure of the barium resonance line are given in table 1. The rather broad peak at about 167
Volume
16, number
1
OPTICS COMMUNICATIONS
January
1976
Table 1 Isotopic-hyperfine shifts in the barium 6s’ ’ So-6s6p ’ PI resonance from mass analysis of laser deflected components of an atomic beam. Error in shift assignment is 5 8 MHz. Shifts marked by an asterisk (*) are taken from ref. 121. Shift (MHz)
Mass number
Fo
Fl
138
0
1
0
137
_3 2
I
56
137
3 2
2 2
280
137
3 z
;
535 *
136
0
I
128
135
3 2
s
135
3 i
3 ii
322
135
3 5
I 5
525 *
134
0
1
138‘
.-
used by Rasmussen et al. [2] allows accurate shifts to be assigned to isotopes with non-zero nuclear spin only. For barium only shifts for odd isotopes are measured. * These lines Irave been resolved using enriched samples by Jackson and Duong Hong Tuan [ 31.
168
\.
/
\
.. I.1
I
I,
I
I .
I,
120
130 MHz in fig. 2 is seen to be a combination of three lines which could only be resolved from the mass spectrometer data. The mass spectrometer data is given in fig. 3 for an averaging time of 20 sec. This is the first experimental resolution of these peaks without the use of enriched samples. They are 135BaF= $_at 120 MHz, 136Ba at 128 MHz and 134Ba at 138 MHz. It is seen that the partially resolved peak in fig. 2 nearest the 138Ba resonance is a hyperfine component of 137Ba at 56 MHz. This was also the strongest of the three 137Ba peaks as observed on both optical and mass spectrometer data and can therefore be assigned as F = s. The F = f transition occurs at 280 MHz and the weak F = i occurs near 530 MHz. (Rasmussen et al. [2] assign a shift of about 535 MHz to the latter transition I). The remaining components, 135 Ba F = : and 135Ba F = i occur at 322 and 530 MHz, respectively. (The !atter is closer to 535 MHz t.) These results are in good agreement with those of recent investigators [2,3] *. The accuracy of our experiment was limited by the stability of the mass analyzer electronics. More modt Note that the method
i.
40
80
120
160
200
MHZ
Fig. 3. Mass peak height as a function of frequency for mass 135 (top), 136 (middle) and 134 (bottom). The solid line is a gaussian which was fitted to the data for this particular run.
ern instruments quote an order of magnitude better stability. With a larger number of runs or longer averaging times, the absolute error in frequency assignment can be reduced still more. Ultimately, the accuracy is limited by the linearity of the piezoelectric crystals (< 1%) in the optical spectrum analyzer and instrumental drift during the course of a run (< 1 MHz). The power of the present method derives from simultaneous utilization of sensitive mass spectral analysis and unusually high resolution (Avjv 5 lo-*) spectral analysis using radiation sources sufficiently intense to physically displace atomic beams through several beam widths via radiation momentum alone. Even when physical separation is prevented by metastable state accumulation, isotope shifts can still be accurately resolved on the basis of the increased efficiency of the ionizer with the metastable atoms. The increased efficiency is indistinguishable from an increase in the concentration of the absorbing isotope. References [ 11 A.F. Bernhardt, D.E. Deurre, J.R. Simpson and L.L. Wood, Appl. Phys Lett. 25 (1974) 617. 121 W. Rasmussen. R. Schieder, H. Walther, Opt. Comm. 12 (1974) 315. 131 D.A. Jackson and Duong Hong Tuan, Proc. Roy. Sot. (London) A280 (1964) 323 and Proc. Roy. Sot. (London) A291 (1966) 9.
16, number
1
OPTICS COMMUNICATIONS
HIGH RESOLUTION
A.F. BERNHARDT
January
SPECTROSCOPY USING PHOTODEFLECTION
1976
*
*, D.E. DUERRE, J.R. SlMPSON and L.L. WOOD
University of Califortzia Lanlrence I.ivermore Laboratory, Livermore, CA 94550, USA Received
22 July
1975. revised
Isotope separation by used to detcrminc optical 5536 A resonance. Several solved with the technique
manuscript
received
3 September
1975
laser deflection of an atomic beam, combined with simultaneous mass spectroscopy, has been frequency shifts and to assign mass numbers to all components of the Ba 6s’ i So-6s6p ‘I’, components which cannot be resolved optically without the use of enriched samples, were rcdescribed. They are 13’Ba(F= S/2) at 120 MHz, ‘36Ba at 128 Mllz and ‘34Ba at 138 hlH7.
The laser deflection of a single isotopic component of an atomic beam has recently been demonstrated [ 11. The technique relies on the selective absorption of laser beam photons by one isotopic component of an atomic beam to transfer momentum from the laser beam to that component. In a well-collimated atomic beam, the momentum acquired by many absorption and spontaneous re-emission events per atom will be sufficient to deflect the isotopic component out of the original beam. This physical separation permits mass analysis of the deflected beam and highly reliable mass assigntnent of various isotopic hyperfine components. In addition, peaks from different isotopic components which cannot be resolved optically, can be unequivocally resolved from the associated mass spectrometer data. This is true since plot of mass peak height as a function of laser frequency gives a separate absorption curve for each mass number. The experimental set-up has been described previously 111 and is presented in fig. 1. An atomic beam of bar-ium is irradiated at right angles with light from an Ar-pumped cw dye laser. The divergence of the atomic beam along the laser axis is 8 mrad. The dye
Work pcrformcd under the auspices of the U.S. Energy Research and Development Administration. Also I:annic and John Ilertz Foundation l:ellovv at the Dt+ partment of Applied Science. University of California at Davis-Livcrmorc.
CW dye laser
iv =
Quadrupole mall
6 MHz
spectrometer
Detecmn
,I,,+
dmde
Fig. 1. Experimental
set up.
laser (Spectra Physics 270/581) t operates on a solution of 2 X 10e4 M Rhodamine 6G in a 3/l mixture of water and hexafluoro-2-propanol. Its output at the 5536 A resonance of barium is 40 mW, with a fwhm linewidth of 5 6 MHz. Before entering the vacuum ? The research equipment and materials designated by brand name in this paper are for identification purposes only, and do not imply the endorsement of the identified equipment or materials by the University of California or the U.S. Energy Research and Development Administration to the exclusion of any other equipment or materials made by other manufacturers which may be equally well suited to the purposes of the work reported.
Volume
16, number
OPTICS COMMUNICATIONS
1
I
.,a 6.6 -
.4 -
.3 -
2
-
.I -
G\
VI -60
I
I
I
0
60
120
b! 180
240
300
360
1
’
1
420
480
540
LJ
Fig. 2. Optical fluorescence spectrum of the barium 6sz ‘So-6s6p ’ P1 resonance. Frequency shifts of the various isotopic-hyperfine components are referred to the center of the ‘38Ba resonance.
system, the laser beam is expanded by a factor of 10 through a cylindrical telescope. Irradiance along the atomic beam is approximately 44 mW/cm2. Radiation scattered by the atomic beam is detected by a photomultiplier tube. A trace of PM output as a function of optical frequency is presented in fig. 2. Six fluorescence peaks are discerned. The largest peak is identified with 138Ba, the most abundant (72%) barium isotope. Shifts are referred to the center of this peak. Twenty centimeters above the base of the irradiation region, a 4 mm wide slit divides the atomic beam, passing only the deflected portion. Directly behind it lies the ionizer of a quadrupole mass spectrometer (EAI Quad 200). The atomic beam enters the ionizer perpendicular to the axis of the quadrupole unit. Barium atoms in the irradiation region absorb and reradiate on the 5536 a transition until decay to the metastable 6s5d 1D2 state occurs from the 6s6p 1P, state. Atoms in the 1 D2 state cease to absorb and cannot be further deflected. Metastable state accumulation is nearly complete. When the laser is tuned to the 138Ba resonance, the upper portion of the atomic beam (that farthest from the beam source) becomes very faint in resonantly scattered 5536 a light. The bottom portion, by contrast, is extremely bright. (This effect is not due to Doppler shift as a result of deflection, because when a second laser operating at
January
1976
5826 a is used to depopulate the metastable state via excitation to the 5d6p 1PI state, the upper portion of the beam does not diminish in intensity of scattered 553.5 a radiation.) Thus most deflected atoms enter the ionizer of the mass spectrometer in the metastable 6sSd ID2 state. The electron energy in the ionizer was set to 10 eV. Such a low energy had the advantage of eliminating hydrocarbon background. It also made the ionizer about twice as efficient in generating ions from metastable atoms than from ground state atoms. The mass spectrometer output was filtered to eliminate r.f. pickup, amplified using a Tektronix 1A7 plug-in, and averaged by means of a Princeton Applied Research TDH9 Waveform Eductor. In this manner, atom densities as low as 3 X lo4 atoms/cc could be reliably detected. A quadrupole unit of more recent design could improve this figure. The frequency standard for the experiment was a Spectra Physics model 470 Optical Spectrum Analyzer with a free spectral range of 2 GHz and resolution of 6 MHz at 5536 8. For stability, the analyzer had been temperature-stabilized and sealed hermetically to prevent longer term drift due to changes in atmospheric pressure. The instrument did not drift observably (< 1 MHz) during the course of experimentation: its “zero-setting” was checked at five to ten minute intervals against the 13*Ba resonance peak. Data was taken in the following manner: the frequency of t‘he laser was changed manually in 8 MHz steps. After averaging for 10 (or in one run, 20) seconds, a picture of an oscilloscope trace of the output of the TDH9 was taken. Four runs, covering the region between -80 and +640 MHz, were made in this way. The height of isotope peaks of mass 134 to 137 were measured and plotted against optical frequency. Electronic instability of the mass analyzer at low signal levels gave some output amplitude uncertainties. Some of this uncertainty was removed by averaging the four runs so that, except for the weak F= i component of the 135Ba and 137Ba peaks, the absolute error is 2 6 MHz. The latter peaks were too close to the sensitivity limit of the mass spectrometer to even be this accurate and the data merely confirms their presence under the peak at 530 MHz in fig. 2. Center frequencies for all peaks in the isotopic hyperfine structure of the barium resonance line are given in table 1. The rather broad peak at about 167
Volume
16, number
1
OPTICS COMMUNICATIONS
January
1976
Table 1 Isotopic-hyperfine shifts in the barium 6s’ ’ So-6s6p ’ PI resonance from mass analysis of laser deflected components of an atomic beam. Error in shift assignment is 5 8 MHz. Shifts marked by an asterisk (*) are taken from ref. 121. Shift (MHz)
Mass number
Fo
Fl
138
0
1
0
137
_3 2
I
56
137
3 2
2 2
280
137
3 z
;
535 *
136
0
I
128
135
3 2
s
135
3 i
3 ii
322
135
3 5
I 5
525 *
134
0
1
138‘
.-
used by Rasmussen et al. [2] allows accurate shifts to be assigned to isotopes with non-zero nuclear spin only. For barium only shifts for odd isotopes are measured. * These lines Irave been resolved using enriched samples by Jackson and Duong Hong Tuan [ 31.
168
\.
/
\
.. I.1
I
I,
I
I .
I,
120
130 MHz in fig. 2 is seen to be a combination of three lines which could only be resolved from the mass spectrometer data. The mass spectrometer data is given in fig. 3 for an averaging time of 20 sec. This is the first experimental resolution of these peaks without the use of enriched samples. They are 135BaF= $_at 120 MHz, 136Ba at 128 MHz and 134Ba at 138 MHz. It is seen that the partially resolved peak in fig. 2 nearest the 138Ba resonance is a hyperfine component of 137Ba at 56 MHz. This was also the strongest of the three 137Ba peaks as observed on both optical and mass spectrometer data and can therefore be assigned as F = s. The F = f transition occurs at 280 MHz and the weak F = i occurs near 530 MHz. (Rasmussen et al. [2] assign a shift of about 535 MHz to the latter transition I). The remaining components, 135 Ba F = : and 135Ba F = i occur at 322 and 530 MHz, respectively. (The !atter is closer to 535 MHz t.) These results are in good agreement with those of recent investigators [2,3] *. The accuracy of our experiment was limited by the stability of the mass analyzer electronics. More modt Note that the method
i.
40
80
120
160
200
MHZ
Fig. 3. Mass peak height as a function of frequency for mass 135 (top), 136 (middle) and 134 (bottom). The solid line is a gaussian which was fitted to the data for this particular run.
ern instruments quote an order of magnitude better stability. With a larger number of runs or longer averaging times, the absolute error in frequency assignment can be reduced still more. Ultimately, the accuracy is limited by the linearity of the piezoelectric crystals (< 1%) in the optical spectrum analyzer and instrumental drift during the course of a run (< 1 MHz). The power of the present method derives from simultaneous utilization of sensitive mass spectral analysis and unusually high resolution (Avjv 5 lo-*) spectral analysis using radiation sources sufficiently intense to physically displace atomic beams through several beam widths via radiation momentum alone. Even when physical separation is prevented by metastable state accumulation, isotope shifts can still be accurately resolved on the basis of the increased efficiency of the ionizer with the metastable atoms. The increased efficiency is indistinguishable from an increase in the concentration of the absorbing isotope. References [ 11 A.F. Bernhardt, D.E. Deurre, J.R. Simpson and L.L. Wood, Appl. Phys Lett. 25 (1974) 617. 121 W. Rasmussen. R. Schieder, H. Walther, Opt. Comm. 12 (1974) 315. 131 D.A. Jackson and Duong Hong Tuan, Proc. Roy. Sot. (London) A280 (1964) 323 and Proc. Roy. Sot. (London) A291 (1966) 9.