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Broadening of level-crossing signals in 52P32 state of 87Rb by collisions with argon
Volume 43A, number 3
PHYSICS LEl-l-ERS
12 March 1973
BROADENING OF LEVEGCROSSING SIGNALS IN S2P,!, STATE OF *‘Rb BY COLLISIONS WITH ARGON* M. UJKASZEWSKI and A. SIERADZAN Institute of Experimental Physics, University of Warsaw, 00481 Warsaw, Poland Received 16 January 1973 The effect of collisions with argon on the widths of level-crossing signals in S2P3,2 state of *‘Rb was studied experimentally. The results were compared with the theory of Okunevich and Perel and certain disagreement was found.
Depolarization of excited atomic states by thermal collisions has been studied by various methods [ 11. In the case of alkali metal atoms broadening of Hanle-effeet signals [2] and D2 optical pumping effects [3] were used most frequently for this purpose. In both cases the interpretation of experimental data is complicated by the presence of the hyperfme structure of the levels. A theoretical study of the collisional relaxation in 2P3/2 states of alkali metal atoms including the effect of nuclear spin was published recently by Okunevich and Perel [4] . It was established that the nuclear spin of the alkali atom is not affected directly by the collisions because the time of collision is small compared to the reciprocal hyperfme structure frequency. In the case when the time of collision is large compared to the reciprocal fine structure frequency (which is the case of heavy alkalis - rubidium and cesium) the total electronic angular momentum J is reoriented in collisions. In consequence the relaxation in the 2P3,2 state is described by 2.J t 1 = 4 independent parameters 7i (i = 0, 1, 2, 3), being the relaxation rates of the multipole components of polarization of the state. The effect of collisions on the observables defined with respect to (F, mF) representation of atomic states can be deduced by making use of the rules of vector addition of the angular mo menta. Okunevich and Perel calculated the values of yr’s and a reduced cross-section s for collisions of Rb and Cs with noble gases assuming that the collisional interaction is of the Van der Waals type. They also suggested that the most direct information on the validity of the theory should be obtained from a * Work supported in part by the Institute of Physics of the Polish Academy of Sciences.
study of collisional broadening of the individual (nonzero field) level-crossing signals, because in that case the broadening can be described by a single cross-section depending on 7;s and s. In the present letter we report the first results of the experiment intended to study the effect of collisions with noble-gas atoms on the width of the nonzero field level-crossing signals in the resonance 2P3,2 excited states of Rb and Cs. Isotope 87 of rubidium is most favourable for such study because in that case it is possible to observe the broadening of three first Am = 2 crossings without difficulties arising from overlapping of the signals, while for 85Rb and Cs this is possible only for the first one. We have used the experimental set-up typical for level-crossing experiments. Exciting light beam from electrodeless rubidium lamp was linearly polarized and directed along the static magnetic field. Rubidium87 vapour and argon were contained in a small Pyrex cell. The static magnetic field was modulated at low frequency. The resonance fluorescence was observed at right angles to the magnetic field. An alternating component in the photomultiplier current was detected by means of a lock-in nanovoltmeter. The level-crossing curves were registered in a strip-chart recorder. The pressure of argon in the cell was varied in the region O-l torr, and was measured by means of a Pirani gauge. Several curves were recorded for each pressure and the mean values of the widths of the signals were determined. The experimental points were fitted to a straight line by the least-squares method. The slope of the line gives the relaxation rates Y‘f m’lP* The energies of the considered levels in function of the magnetic field and the coefficients of expansion 227
Volume 43A, number 3
PHYSICS LETTERS
of the wave functions of the levels in the field of crossing on the zero-field wave functions were calculated in the formalism of Kaperlewski and Rosinski [S] . The most recent values of a [6] , b andgJ [7] were used. The calculations show that it follows from the theory of Okunevich and Perel that the relaxation rates for the considered crossigns, (2, -2) X (1, 0), (3, -2) X (2, 0) and (3, -3) X (2, -1) are equal to 0.63, 0.85 and 0.97 respectively in units of nGs (n is the concentration of the foreign gas, V is the mean relative velocity, s is the reduced cross-section). For 87Rb-Ar pair these rates amount to 19,25 and 29 X lo6 s-l torr-l respectively. The results of the measurements are in certain disagreement with the above values. We have obtained yfi,-,,/p equal to 39(4), 34(6) and 38(4) X lo6 s-l torr-l respectively for the three crossings studied. It appears that the relaxation rates are virtually the same for the three crossings and are larger than the ones obtained from the theory. The reason for the discrepancy is not clear to us. We hope that the measure-
228
12 March 1973
ments for other noble gases and for 85 Rb and Cs, which are in progress, allow us to obtain more definite conclusions. The authors wish to thank Professor T. Skalinski for his kind interest shown in this work.
References [ 1 ] A. Omont, in: Physics of the one- and two-electron atoms (Norht-Holland, 1969) p. 777. [2] A. Gallagher, Phys. Rev. 157 (1967) 68; G.V. Markova, G.I. Khvostenko and M.P. Chaika, Optika i Spektroskopiya 23 (1967) 835. [3] M. Elbel, A. Koch and W. Schneider, Z. Physik 255 (1972) 14, and references therein. [4] A.I. Okunevich and V.I. Perel, Zh. Eksp. I Teor. Fiz. 58 (1970) 666, Soviet Phys. JETP 31 (1970) 356. [S] J. Kapelewski and K. Rosifiski, Acta Phys. Polon. 28 (1965) 177. [6] H.A. Schiissler, Z. Physik 182 (1965) 289. [7] G. Belin and S. Scanberg, Physica Scripta 4 (1971) 269.
PHYSICS LEl-l-ERS
12 March 1973
BROADENING OF LEVEGCROSSING SIGNALS IN S2P,!, STATE OF *‘Rb BY COLLISIONS WITH ARGON* M. UJKASZEWSKI and A. SIERADZAN Institute of Experimental Physics, University of Warsaw, 00481 Warsaw, Poland Received 16 January 1973 The effect of collisions with argon on the widths of level-crossing signals in S2P3,2 state of *‘Rb was studied experimentally. The results were compared with the theory of Okunevich and Perel and certain disagreement was found.
Depolarization of excited atomic states by thermal collisions has been studied by various methods [ 11. In the case of alkali metal atoms broadening of Hanle-effeet signals [2] and D2 optical pumping effects [3] were used most frequently for this purpose. In both cases the interpretation of experimental data is complicated by the presence of the hyperfme structure of the levels. A theoretical study of the collisional relaxation in 2P3/2 states of alkali metal atoms including the effect of nuclear spin was published recently by Okunevich and Perel [4] . It was established that the nuclear spin of the alkali atom is not affected directly by the collisions because the time of collision is small compared to the reciprocal hyperfme structure frequency. In the case when the time of collision is large compared to the reciprocal fine structure frequency (which is the case of heavy alkalis - rubidium and cesium) the total electronic angular momentum J is reoriented in collisions. In consequence the relaxation in the 2P3,2 state is described by 2.J t 1 = 4 independent parameters 7i (i = 0, 1, 2, 3), being the relaxation rates of the multipole components of polarization of the state. The effect of collisions on the observables defined with respect to (F, mF) representation of atomic states can be deduced by making use of the rules of vector addition of the angular mo menta. Okunevich and Perel calculated the values of yr’s and a reduced cross-section s for collisions of Rb and Cs with noble gases assuming that the collisional interaction is of the Van der Waals type. They also suggested that the most direct information on the validity of the theory should be obtained from a * Work supported in part by the Institute of Physics of the Polish Academy of Sciences.
study of collisional broadening of the individual (nonzero field) level-crossing signals, because in that case the broadening can be described by a single cross-section depending on 7;s and s. In the present letter we report the first results of the experiment intended to study the effect of collisions with noble-gas atoms on the width of the nonzero field level-crossing signals in the resonance 2P3,2 excited states of Rb and Cs. Isotope 87 of rubidium is most favourable for such study because in that case it is possible to observe the broadening of three first Am = 2 crossings without difficulties arising from overlapping of the signals, while for 85Rb and Cs this is possible only for the first one. We have used the experimental set-up typical for level-crossing experiments. Exciting light beam from electrodeless rubidium lamp was linearly polarized and directed along the static magnetic field. Rubidium87 vapour and argon were contained in a small Pyrex cell. The static magnetic field was modulated at low frequency. The resonance fluorescence was observed at right angles to the magnetic field. An alternating component in the photomultiplier current was detected by means of a lock-in nanovoltmeter. The level-crossing curves were registered in a strip-chart recorder. The pressure of argon in the cell was varied in the region O-l torr, and was measured by means of a Pirani gauge. Several curves were recorded for each pressure and the mean values of the widths of the signals were determined. The experimental points were fitted to a straight line by the least-squares method. The slope of the line gives the relaxation rates Y‘f m’lP* The energies of the considered levels in function of the magnetic field and the coefficients of expansion 227
Volume 43A, number 3
PHYSICS LETTERS
of the wave functions of the levels in the field of crossing on the zero-field wave functions were calculated in the formalism of Kaperlewski and Rosinski [S] . The most recent values of a [6] , b andgJ [7] were used. The calculations show that it follows from the theory of Okunevich and Perel that the relaxation rates for the considered crossigns, (2, -2) X (1, 0), (3, -2) X (2, 0) and (3, -3) X (2, -1) are equal to 0.63, 0.85 and 0.97 respectively in units of nGs (n is the concentration of the foreign gas, V is the mean relative velocity, s is the reduced cross-section). For 87Rb-Ar pair these rates amount to 19,25 and 29 X lo6 s-l torr-l respectively. The results of the measurements are in certain disagreement with the above values. We have obtained yfi,-,,/p equal to 39(4), 34(6) and 38(4) X lo6 s-l torr-l respectively for the three crossings studied. It appears that the relaxation rates are virtually the same for the three crossings and are larger than the ones obtained from the theory. The reason for the discrepancy is not clear to us. We hope that the measure-
228
12 March 1973
ments for other noble gases and for 85 Rb and Cs, which are in progress, allow us to obtain more definite conclusions. The authors wish to thank Professor T. Skalinski for his kind interest shown in this work.
References [ 1 ] A. Omont, in: Physics of the one- and two-electron atoms (Norht-Holland, 1969) p. 777. [2] A. Gallagher, Phys. Rev. 157 (1967) 68; G.V. Markova, G.I. Khvostenko and M.P. Chaika, Optika i Spektroskopiya 23 (1967) 835. [3] M. Elbel, A. Koch and W. Schneider, Z. Physik 255 (1972) 14, and references therein. [4] A.I. Okunevich and V.I. Perel, Zh. Eksp. I Teor. Fiz. 58 (1970) 666, Soviet Phys. JETP 31 (1970) 356. [S] J. Kapelewski and K. Rosifiski, Acta Phys. Polon. 28 (1965) 177. [6] H.A. Schiissler, Z. Physik 182 (1965) 289. [7] G. Belin and S. Scanberg, Physica Scripta 4 (1971) 269.