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Zeeman effects in the hyperfine structure of atomic iodine photodissociation laser emission
Volume 13, number 5
CHEMICAL PHYSICS
ZEEMAN ATOMIC
EFFECTS
IN THE
1 April 1972
LETTERS
HYPERFINE
IODINE PHOTODiSSOCIATION
STRUCTURE
OF
LASER EMISSION*
W.C. HW_4NG and J.V.V. KASPER** Departnzerrt of Chemistry ***, University of California, Los Angeles, California 90024. US.4
Received 17 January 1972
Hyperfine structure has been obseervcd in laser emission from CFaI and CzFjI photodissociation lasers. Constant magnetic fields affect the time behavior of the emission by changing the relative gains of the hypcrfme transitions. Time-varying fields usually present in photodissociation lasers further complicate the emission.
Recently there has been increased interest in the kinetics and spectroscopy of the halide photodissociation lasers. The effects of temperature [ I ] and con-
centration of 1, [2] on the quenching of 2P,,2 + +3/2 atomic iodine iaser emission have been studied. Other investigations [3,4] have presented evidence for a secondary pumping process in the CF31 laser. Computer simulations [S] of CF,I laser emission have been compared to experimental results in order to identify the most important processes. The IBr laser (where the emission is from aP,,a Br) has been reinvestigated [6] with the goal of determining details of the recombination mechanism. Laser emission has been obtained from CF,Br and zero-field hyperfine splitting of the atomic bromine transition was observed [71* Hyperfine structure in atomic iodine laser emission has to date been almost entirely neglected. Using a SiSAM spectrometer, resolution spectrum
Verges i8] obtained a highof spontaneous emission from
2P,,2 iodine atoms excited by an electrodeless disThe six zero-field hyperfine transitions were
charge.
* Acknowledgement leum
Research
is made
to the donors
of the Petro-
Fund, administered by the American
to the Research Committee, UCLA, and to NASA (NGROS-007-003) for support of this re-
Chemical Society, search. *f Alfred P. Sloan ***
Contribution
Foundation
FeUow.
No. 2939 from UCLA.
into four isolated lines and one blended pair. Volkov and Zubarev [9] have observed two hyperfine transitions separated by 0.47 cm-1 in photographs of Fabry-Perot interference fringes from C3F71 laser emission. WC have independently observed hyperfrne structure in emission from CF,I and C,F,I lasers and have studied the detailed time behavio; of the hyperfine emissions in both the presence and absence of applied magnetic fields. The time behavior can be partially explained by Zeeman effects on the widths and strengths of the hypertine lines. Our studies indicate that in most investigations published to date, time-varying magnetic fields produced by the commonly used adjacent flash tubes complicated the laser emission. The photodissociation laser apparatus used in this study is similar to that described by Campbell and Kasper [7, lo] _The pumping flash is produced by a capacitive discharge through 20 torr of fresh xenon in the annulus between an inner 7-mm-i.d. quartz laser tube and an outer 17.mm-i.d. quartz tube. The discharge current returns through a pair of 4-cm wide copper strips which run the length of the laser tube and which are symmetrically located 6 cm on either side of it. Since the magnetic field inside a uniform cylindrical current sheath is zero, a totally homogeneous discharge in the annulus produces no magnetic field within the laser tube. No inhomogeneities were visually detected in 100-J discharges through resolved
511
Volume
13, number
5
CHEMICAL
Fresh xenon. The magnetic field at the peak of the discharge current is estimated to be at most 40 G [lo]. The flash, monitored by an RCA lP28 photomultiplier tube through a Jarrell-Ash 4-m grating monochromator, has an S-,usec risetime and a 7-Dsec halfwidth at 265 nm. The total laser emission was observed by reflecting a small fraction of the laser radiation to a Mutlard ORP-10 room temperature, photoconducting InSb detector. The major portion of the laser radiation was focused onto the entrance slit of a Jarrell-Ash 1.0-m Czerny-Turner moncchromatqr with a spectral slit of 0.15 cm-l at 1.315 urn. Radiation transmitted by the monochromator was detected by an RCA 7102 photomultiplier which has an S-l photocathode. A magnetic field parallel to the laser axis was produced by an “axial” electromagnet which consisted of 480 coaxial coils of lo-gauge copper wire wrapped around 46 cm of the insulated outer quartz tube. A “transverse” electromagnet was constructed of 76 longitudinal loops of l6-gauge wire wound on a wooden form placed over the laser. A current pdse with a 3.5-msec half-width and with peak currents as large as 100 A generated transverse fields up to 1100 G or atin1 fields up to 1300 G. The current was constant (near its peak value) to better than 0.1% for the 50.psec duration of the experiments. Furthennore, the fields were estimated to vary less than 5% along the central 80% of’the 500-n active region. The zero-field hyperfine sublevels* of at.omic iodine and the sh allowed transitions (A.! = +l, W = 0, 2 !) are shown in fig. la. The splittings for the 2P,,, level have been accurately determined by Jaccarino et al. [ 1 l] using atomic beam magnetic resonance spectroscopy. The 2P1,2 splitting (0.692 0.02cm- ‘) was obtained from analysis of Verges’ published spectrum [8]. The theoretical zero-field spectrum is shown in fig. 1b with proper relative separations and line strengths [12]. We observed two lines separated by 0.45 I 0.06 cm-I in !aser emission from CF3I and C2F,I with no applied magnetic field. This separation is the same as that reported by Volkov and Zubarev (91. We conclude that each of the two lines is the transition of greatest line strength which originates
* Level refers to the set of alI states with the same electronic configuration and value of J.SubMel refers to that set of mF states in a given J level which have the same value of F. 512
1 April 1972
PHYSICS LETTERS
4 * l!
FREQUENCY
1
O.l42cm-’ O.OGcm* a.025 cm-’
-
Fig. 1. Zero-field hyperfine
structure of atomic ‘2iI. (ni Hyperfine levels, separations, and allowed transitions. (6) Theoretical spectrum and relative Line strengths, on a given upper F sublevel; specifical!y, these lines are the F = 3 + 4i and the F = 2 -+ 2 transitions (see
fig. 1). Although the F = 3 + 3 transition has the Qrne line strength as the F= 2 + 2 transition, it is not observed at zero magnetic field. The absence of this transition requires that relaxation among the hyperfine sublevels of the 2P3,2 level is rapid on the time scale of the laser emission. The time behavior of the zero-field CF,I laser emission is shown in fig. 3_a. On the left side of the figure is the total laser emission. The right side shows the emission when the monochromator is set at the peaks of either the F = 3 + 4 (upper line) or the F = 2 -+ 2 (lower line) transition. The F = 2 + 2 emission initiates 2 I.csecIater and is much weaker than the F = 3 -+ 4 emission. The delay implies that the peak gain of the F = 2 + 2 line is less than that of the F - 3 + 4 line. This is consistent with the zero-field line strengths assuming that there is a Boltzmann distribution among the hyperfine sublevels and that these lines are resolved and have equal line wide&s. Both the weakness and the short duration of the F = 2 + 2 transition indicate that it is barely above threshold. In fact, the ? For
F = 3 -+ 4, read 2P,,2(F=
3) + 2P3,2(F=
4).
Volumet3.number5
CHEMKAL
PHYSlCS LETTERS
1 April 1972
emission versus time for 20 torr CF& lnteasity in arbitrary units. Total emission on left. (a) 250-J inhomogencous discharge. F = 3 --, 4 line (upper) and F = 2 --f2 line (lowor) on right. (bj 300-J homogeneous discharge with wire current return 1.5 cm from the laser axis, F = 3 4 4 line on Fig. 3. Lsscr
right. Fig. 2.Laser emission versus time for 20 torr CFzi at 100-J flash energy. Intensities in arbitrary units. Total emission on left. F = 3 + 4 line (upper) and F = 2 * 2 line (Iower) on right. (a) Applied magnetic field = 0. (b) Transverse field = 500 G. (c) Transverse field = 1 LOOG. (d) Axial field = 500 G.
emission is sometimes completely absent. That this transitfon is close to threshold can be attributed to rapid reIaxation between the hyperfine sublevels of the 2Pr,z IeveI. The effects of constant transverse magnetic Field on the CF$ laser emission can be seen in figs. 2b and 2c which show the emission at Gelds of 500 and I IO0 G, respectively. One effect is that initiation of the total laser emission is delayed relative to that of the zero-fiefd Iarer emission. The magnetic field removes the degeneracy of the r?zF states of 3 given F sublevef and mixes* states of different E;. Consequentiy, it broadens the hypertjne Iines, lowers the peak gain, and delays laser initiation.
Furthermore,
the
laser emission in the presence of a 500-G transverse field consists entireiy of the F = 2 -+ 2 line (see fig. Zb). 7Ire complete absence of the F = 3 * 4 tine implies both that the peak gain of the F= 2 + 2 line is now greater than that of the F= 3 + 4 line and that rapid relaxation occurs among the hyperfine sublevels of t& 2pt12 and of the 2P3,2 levels. Calculations I1 3) of transition intensities and frequencies show that the F = 2 --f 2 line does indeed have the higher peak gain
in a 500-G transverse field. Finally, at I100G (see fig. 7,~) both the F =2+2andF=3+4emissions are present but weak, and their sum is less than the totat laser emission. The magnetic field has app~res~t~~ shifted some of the emission intensity to the F = 3 -+ 3 line; the intensity at the peak of this line was not mnnitored. A,xial fields up to 1300 G affect the CF,I laser emission in a manner similar to that of transverse fields with one major exception. In a 500-G axial fietd, the F = 3 + 4 line is not only present, but it initiates at essentially the same time as the F = 2 + 2 line (see fig. 2d). In axial fields only n transitions (dtzF = + 1 for magnetic dipole transitions) are allowed; in transverse fields both u (&RF = 0) and z transitions
f!3] , Since u and pi transitj~ns
havedifferent line strengths in the presence of mngnetic fields, the relative gains of the various lines will in general depend on the direction of the applied fields. Our results indicate that for a 500-G axial field the pertic gains of the F = 3-+4andF=2+2lines
are simifar; for a 500-Gtransverse field the peak gain of the F = 2 + 2 line is greater than that of the F = 3 + 4 line. With applied magnetic fields the behavior of the hyperfine laser emission from CaFgI is simiiar to that from CF,I. However, under identical conditions in a 500-G transverse field, the F = 3 + 4 line from this precursor
* Even though F is not a gocd quantum number in the prosence of a magnetic field, we continue to usz F for consistency of notation
are allowed
although
delayed
and weak was present.
We tentatively attribute this difference in behavior to different line widths in the two laser systems. When the magnetic field is not constant but varies 513
Volume
13, number 5
CHEMICAL PHYSICS LETTERS
with time, the behavior of the laser emission becomes even more complicated. Time-varying transverse fields up to several thousand gauss can be generated by discharges in adjacent flash tubes and by strongly inhomogeneous discharges in concentric (or annular) flash tubes. (A 10&A current produces a 1000-G magnetic field at a distance of 2 cm.) Our concentric flash system produced a visibly inhomogeneous discharge when xenon was left in the flash tube for 24 hours. Fig. 3a shows the laser emission resulting from a 250-J inhomogeneous flash. The F = 2 + 2 line initiates first and continues for 3.5 psec at which time it suddenly terminates. The F = 3 + 4 line begins lasing essentially when the F = 2 + 2 line terminates and continues untii the end of the laser emission. Again the total laser
emission is delayed relative to zero-rield emission. We simulated the field produced by an adjacent flash tube by replacing the copper return strip; with a single 12gauge copper wire parallel to the laser tube. With the wire 1.5 cm from the center of the laser tube, the F = 3 4 4 emission produced by a 300-J homogeneous discharge 113s the oscillatory time behavior shown in fig. 3b. The total iaser emission is delayed and slight intensity fluctuations which parallel the F = 3 + 4 oscillations are observed. With the return wire moved to a distance of 7.5 cm from the laser axis, both the F= 3 + 4 emission and the total emission exhibit behavior characteristic of zero-field emission. Clearly, time-varying magnetic fields when produced by flash discharges markedly affect the hyperfine emission behavior. Furthermore, under some conditions the emission switches from the F = 2 + 2 to the F= 3 + 4 line so rapidly that a second spike is observed in the total emission (see fig. 3a). This spike corresponds to simultaneous strong emission from both of these lines. Such switching may well be the cause of the second major spike and the subsequent slight variation in Iaser intensity indicated in fig. I of Andreeva et al. 1141. We have shown that iodine photodissociation lasers are more complex than was previously realized. Any model for zero-Geld lasers should include the six hyperfme sublevels, the six allowed transitions, and relaxation among the hyperflne sublevels of a given J level. Studies of laser behavior in constant magnetic fields must consider the Zeeman spiitting of the six zero-field hyperfine sublevels instead of just.the twc
J levels. The relative gains of the hyperfine lines are 514
1 April 1972
affected both by the Zeeman splitting and by magnetic-field induced mixing between states of different F. Most flash lamps which are used in photodissociation laser studies generate significsnt time-varying magnetic fields. The resultant time-varying transition frequencies and relative per’- gains greatly complicate the laser emission and its analysis. Studies of Zeeman effects on the hyperfine structure in spontaneous and stimulated emission are being extended using a Fabry-Perot interferometer. Computer simulation of the laser emission is being performed for a variety of experimental conditions in order to test different kinetic models. These investigations will provide information regarding spin-orbit relaxation, optical line widths, and the importance of secondary chemica! reactions. Furthermore, they should elucidate the nature and role of hyperfine relaxation in halide photodissociation lasers.
We are grateful to J.D. Campbell, R.F. Heidner III, A.T. Pritt Jr., and Professors K.D. Bnyes, A.U. Hazi and E.Y. Wong for helpful discussions. References Ill V.Yu.Zalesski: (1970)
and E.I.Moskalev,
Scviet Phys. JEW
30
1019.
[21 I.bl.Belousova, O.B.Danilov, N.S.Kladovikoua and I.L. Yachnev, Soviet Phys. ‘Tech. Phys. 15 (197 1) 1212. i31 T.L.Andreeva, V.I.XIalyshev, A.I.hlaslov, G.YltSolov’yev and V.N.Sorokin, Kratkiye Soobshch. PO E:iz. 10 (1970) 71. 141 P.Gensel, K.Hohla and K.L.Kompa, Appl. Phys. Letters
18 (1971) 48. [51 D.E.O’Brisn and J.R.Bowen, J. Appl. Phys. 42 (1971) 1010. 161 V.A.Dudkin, I.N.Knyxzev and V.I.hIalyshcv, Kratkiye Soobshch. PO Fiz. 5 (1970) 32. 171 J.D.Campbell and J.V.V.Kasper, Chem. Phys Letters 10 (197 1) 436. [81 J.Verges, Spectrochim. Actn 24B (1959) 177. 191 V.N.Volkov and I.G.Zubaxv, Kratkiye Soobshch. PO I%. IO (1970) 10. IlO1 J.D.Campbell and J.V.V.Kasper, to be published. 1111 V.Jaccarino, J.G.King, R.A.Suttcn and H.H.Stroke, Phys Rev. 94 (1954) 1798. [l?l E.U.Condon and G.H.Shortleg, The theory of atomic spectra (Cambridge Univ. Press, London, 1967). 1131 A.U.Hazi, J.V.V.Kasper and A.T.Pritt Jr., to be pub lished. I141 T.L.Andrueva, V.A.Dudkin, V.I.h!a.lyshev,G.V. hfikha~ov and V.N.Sorokin, Soviet Phys. JETP 22 (1966) 969.
CHEMICAL PHYSICS
ZEEMAN ATOMIC
EFFECTS
IN THE
1 April 1972
LETTERS
HYPERFINE
IODINE PHOTODiSSOCIATION
STRUCTURE
OF
LASER EMISSION*
W.C. HW_4NG and J.V.V. KASPER** Departnzerrt of Chemistry ***, University of California, Los Angeles, California 90024. US.4
Received 17 January 1972
Hyperfine structure has been obseervcd in laser emission from CFaI and CzFjI photodissociation lasers. Constant magnetic fields affect the time behavior of the emission by changing the relative gains of the hypcrfme transitions. Time-varying fields usually present in photodissociation lasers further complicate the emission.
Recently there has been increased interest in the kinetics and spectroscopy of the halide photodissociation lasers. The effects of temperature [ I ] and con-
centration of 1, [2] on the quenching of 2P,,2 + +3/2 atomic iodine iaser emission have been studied. Other investigations [3,4] have presented evidence for a secondary pumping process in the CF31 laser. Computer simulations [S] of CF,I laser emission have been compared to experimental results in order to identify the most important processes. The IBr laser (where the emission is from aP,,a Br) has been reinvestigated [6] with the goal of determining details of the recombination mechanism. Laser emission has been obtained from CF,Br and zero-field hyperfine splitting of the atomic bromine transition was observed [71* Hyperfine structure in atomic iodine laser emission has to date been almost entirely neglected. Using a SiSAM spectrometer, resolution spectrum
Verges i8] obtained a highof spontaneous emission from
2P,,2 iodine atoms excited by an electrodeless disThe six zero-field hyperfine transitions were
charge.
* Acknowledgement leum
Research
is made
to the donors
of the Petro-
Fund, administered by the American
to the Research Committee, UCLA, and to NASA (NGROS-007-003) for support of this re-
Chemical Society, search. *f Alfred P. Sloan ***
Contribution
Foundation
FeUow.
No. 2939 from UCLA.
into four isolated lines and one blended pair. Volkov and Zubarev [9] have observed two hyperfine transitions separated by 0.47 cm-1 in photographs of Fabry-Perot interference fringes from C3F71 laser emission. WC have independently observed hyperfrne structure in emission from CF,I and C,F,I lasers and have studied the detailed time behavio; of the hyperfine emissions in both the presence and absence of applied magnetic fields. The time behavior can be partially explained by Zeeman effects on the widths and strengths of the hypertine lines. Our studies indicate that in most investigations published to date, time-varying magnetic fields produced by the commonly used adjacent flash tubes complicated the laser emission. The photodissociation laser apparatus used in this study is similar to that described by Campbell and Kasper [7, lo] _The pumping flash is produced by a capacitive discharge through 20 torr of fresh xenon in the annulus between an inner 7-mm-i.d. quartz laser tube and an outer 17.mm-i.d. quartz tube. The discharge current returns through a pair of 4-cm wide copper strips which run the length of the laser tube and which are symmetrically located 6 cm on either side of it. Since the magnetic field inside a uniform cylindrical current sheath is zero, a totally homogeneous discharge in the annulus produces no magnetic field within the laser tube. No inhomogeneities were visually detected in 100-J discharges through resolved
511
Volume
13, number
5
CHEMICAL
Fresh xenon. The magnetic field at the peak of the discharge current is estimated to be at most 40 G [lo]. The flash, monitored by an RCA lP28 photomultiplier tube through a Jarrell-Ash 4-m grating monochromator, has an S-,usec risetime and a 7-Dsec halfwidth at 265 nm. The total laser emission was observed by reflecting a small fraction of the laser radiation to a Mutlard ORP-10 room temperature, photoconducting InSb detector. The major portion of the laser radiation was focused onto the entrance slit of a Jarrell-Ash 1.0-m Czerny-Turner moncchromatqr with a spectral slit of 0.15 cm-l at 1.315 urn. Radiation transmitted by the monochromator was detected by an RCA 7102 photomultiplier which has an S-l photocathode. A magnetic field parallel to the laser axis was produced by an “axial” electromagnet which consisted of 480 coaxial coils of lo-gauge copper wire wrapped around 46 cm of the insulated outer quartz tube. A “transverse” electromagnet was constructed of 76 longitudinal loops of l6-gauge wire wound on a wooden form placed over the laser. A current pdse with a 3.5-msec half-width and with peak currents as large as 100 A generated transverse fields up to 1100 G or atin1 fields up to 1300 G. The current was constant (near its peak value) to better than 0.1% for the 50.psec duration of the experiments. Furthennore, the fields were estimated to vary less than 5% along the central 80% of’the 500-n active region. The zero-field hyperfine sublevels* of at.omic iodine and the sh allowed transitions (A.! = +l, W = 0, 2 !) are shown in fig. la. The splittings for the 2P,,, level have been accurately determined by Jaccarino et al. [ 1 l] using atomic beam magnetic resonance spectroscopy. The 2P1,2 splitting (0.692 0.02cm- ‘) was obtained from analysis of Verges’ published spectrum [8]. The theoretical zero-field spectrum is shown in fig. 1b with proper relative separations and line strengths [12]. We observed two lines separated by 0.45 I 0.06 cm-I in !aser emission from CF3I and C2F,I with no applied magnetic field. This separation is the same as that reported by Volkov and Zubarev (91. We conclude that each of the two lines is the transition of greatest line strength which originates
* Level refers to the set of alI states with the same electronic configuration and value of J.SubMel refers to that set of mF states in a given J level which have the same value of F. 512
1 April 1972
PHYSICS LETTERS
4 * l!
FREQUENCY
1
O.l42cm-’ O.OGcm* a.025 cm-’
-
Fig. 1. Zero-field hyperfine
structure of atomic ‘2iI. (ni Hyperfine levels, separations, and allowed transitions. (6) Theoretical spectrum and relative Line strengths, on a given upper F sublevel; specifical!y, these lines are the F = 3 + 4i and the F = 2 -+ 2 transitions (see
fig. 1). Although the F = 3 + 3 transition has the Qrne line strength as the F= 2 + 2 transition, it is not observed at zero magnetic field. The absence of this transition requires that relaxation among the hyperfine sublevels of the 2P3,2 level is rapid on the time scale of the laser emission. The time behavior of the zero-field CF,I laser emission is shown in fig. 3_a. On the left side of the figure is the total laser emission. The right side shows the emission when the monochromator is set at the peaks of either the F = 3 + 4 (upper line) or the F = 2 -+ 2 (lower line) transition. The F = 2 + 2 emission initiates 2 I.csecIater and is much weaker than the F = 3 -+ 4 emission. The delay implies that the peak gain of the F = 2 + 2 line is less than that of the F - 3 + 4 line. This is consistent with the zero-field line strengths assuming that there is a Boltzmann distribution among the hyperfine sublevels and that these lines are resolved and have equal line wide&s. Both the weakness and the short duration of the F = 2 + 2 transition indicate that it is barely above threshold. In fact, the ? For
F = 3 -+ 4, read 2P,,2(F=
3) + 2P3,2(F=
4).
Volumet3.number5
CHEMKAL
PHYSlCS LETTERS
1 April 1972
emission versus time for 20 torr CF& lnteasity in arbitrary units. Total emission on left. (a) 250-J inhomogencous discharge. F = 3 --, 4 line (upper) and F = 2 --f2 line (lowor) on right. (bj 300-J homogeneous discharge with wire current return 1.5 cm from the laser axis, F = 3 4 4 line on Fig. 3. Lsscr
right. Fig. 2.Laser emission versus time for 20 torr CFzi at 100-J flash energy. Intensities in arbitrary units. Total emission on left. F = 3 + 4 line (upper) and F = 2 * 2 line (Iower) on right. (a) Applied magnetic field = 0. (b) Transverse field = 500 G. (c) Transverse field = 1 LOOG. (d) Axial field = 500 G.
emission is sometimes completely absent. That this transitfon is close to threshold can be attributed to rapid reIaxation between the hyperfine sublevels of the 2Pr,z IeveI. The effects of constant transverse magnetic Field on the CF$ laser emission can be seen in figs. 2b and 2c which show the emission at Gelds of 500 and I IO0 G, respectively. One effect is that initiation of the total laser emission is delayed relative to that of the zero-fiefd Iarer emission. The magnetic field removes the degeneracy of the r?zF states of 3 given F sublevef and mixes* states of different E;. Consequentiy, it broadens the hypertjne Iines, lowers the peak gain, and delays laser initiation.
Furthermore,
the
laser emission in the presence of a 500-G transverse field consists entireiy of the F = 2 -+ 2 line (see fig. Zb). 7Ire complete absence of the F = 3 * 4 tine implies both that the peak gain of the F= 2 + 2 line is now greater than that of the F= 3 + 4 line and that rapid relaxation occurs among the hyperfine sublevels of t& 2pt12 and of the 2P3,2 levels. Calculations I1 3) of transition intensities and frequencies show that the F = 2 --f 2 line does indeed have the higher peak gain
in a 500-G transverse field. Finally, at I100G (see fig. 7,~) both the F =2+2andF=3+4emissions are present but weak, and their sum is less than the totat laser emission. The magnetic field has app~res~t~~ shifted some of the emission intensity to the F = 3 -+ 3 line; the intensity at the peak of this line was not mnnitored. A,xial fields up to 1300 G affect the CF,I laser emission in a manner similar to that of transverse fields with one major exception. In a 500-G axial fietd, the F = 3 + 4 line is not only present, but it initiates at essentially the same time as the F = 2 + 2 line (see fig. 2d). In axial fields only n transitions (dtzF = + 1 for magnetic dipole transitions) are allowed; in transverse fields both u (&RF = 0) and z transitions
f!3] , Since u and pi transitj~ns
havedifferent line strengths in the presence of mngnetic fields, the relative gains of the various lines will in general depend on the direction of the applied fields. Our results indicate that for a 500-G axial field the pertic gains of the F = 3-+4andF=2+2lines
are simifar; for a 500-Gtransverse field the peak gain of the F = 2 + 2 line is greater than that of the F = 3 + 4 line. With applied magnetic fields the behavior of the hyperfine laser emission from CaFgI is simiiar to that from CF,I. However, under identical conditions in a 500-G transverse field, the F = 3 + 4 line from this precursor
* Even though F is not a gocd quantum number in the prosence of a magnetic field, we continue to usz F for consistency of notation
are allowed
although
delayed
and weak was present.
We tentatively attribute this difference in behavior to different line widths in the two laser systems. When the magnetic field is not constant but varies 513
Volume
13, number 5
CHEMICAL PHYSICS LETTERS
with time, the behavior of the laser emission becomes even more complicated. Time-varying transverse fields up to several thousand gauss can be generated by discharges in adjacent flash tubes and by strongly inhomogeneous discharges in concentric (or annular) flash tubes. (A 10&A current produces a 1000-G magnetic field at a distance of 2 cm.) Our concentric flash system produced a visibly inhomogeneous discharge when xenon was left in the flash tube for 24 hours. Fig. 3a shows the laser emission resulting from a 250-J inhomogeneous flash. The F = 2 + 2 line initiates first and continues for 3.5 psec at which time it suddenly terminates. The F = 3 + 4 line begins lasing essentially when the F = 2 + 2 line terminates and continues untii the end of the laser emission. Again the total laser
emission is delayed relative to zero-rield emission. We simulated the field produced by an adjacent flash tube by replacing the copper return strip; with a single 12gauge copper wire parallel to the laser tube. With the wire 1.5 cm from the center of the laser tube, the F = 3 4 4 emission produced by a 300-J homogeneous discharge 113s the oscillatory time behavior shown in fig. 3b. The total iaser emission is delayed and slight intensity fluctuations which parallel the F = 3 + 4 oscillations are observed. With the return wire moved to a distance of 7.5 cm from the laser axis, both the F= 3 + 4 emission and the total emission exhibit behavior characteristic of zero-field emission. Clearly, time-varying magnetic fields when produced by flash discharges markedly affect the hyperfine emission behavior. Furthermore, under some conditions the emission switches from the F = 2 + 2 to the F= 3 + 4 line so rapidly that a second spike is observed in the total emission (see fig. 3a). This spike corresponds to simultaneous strong emission from both of these lines. Such switching may well be the cause of the second major spike and the subsequent slight variation in Iaser intensity indicated in fig. I of Andreeva et al. 1141. We have shown that iodine photodissociation lasers are more complex than was previously realized. Any model for zero-Geld lasers should include the six hyperfme sublevels, the six allowed transitions, and relaxation among the hyperflne sublevels of a given J level. Studies of laser behavior in constant magnetic fields must consider the Zeeman spiitting of the six zero-field hyperfine sublevels instead of just.the twc
J levels. The relative gains of the hyperfine lines are 514
1 April 1972
affected both by the Zeeman splitting and by magnetic-field induced mixing between states of different F. Most flash lamps which are used in photodissociation laser studies generate significsnt time-varying magnetic fields. The resultant time-varying transition frequencies and relative per’- gains greatly complicate the laser emission and its analysis. Studies of Zeeman effects on the hyperfine structure in spontaneous and stimulated emission are being extended using a Fabry-Perot interferometer. Computer simulation of the laser emission is being performed for a variety of experimental conditions in order to test different kinetic models. These investigations will provide information regarding spin-orbit relaxation, optical line widths, and the importance of secondary chemica! reactions. Furthermore, they should elucidate the nature and role of hyperfine relaxation in halide photodissociation lasers.
We are grateful to J.D. Campbell, R.F. Heidner III, A.T. Pritt Jr., and Professors K.D. Bnyes, A.U. Hazi and E.Y. Wong for helpful discussions. References Ill V.Yu.Zalesski: (1970)
and E.I.Moskalev,
Scviet Phys. JEW
30
1019.
[21 I.bl.Belousova, O.B.Danilov, N.S.Kladovikoua and I.L. Yachnev, Soviet Phys. ‘Tech. Phys. 15 (197 1) 1212. i31 T.L.Andreeva, V.I.XIalyshev, A.I.hlaslov, G.YltSolov’yev and V.N.Sorokin, Kratkiye Soobshch. PO E:iz. 10 (1970) 71. 141 P.Gensel, K.Hohla and K.L.Kompa, Appl. Phys. Letters
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