Laser excitation of 6s6d 63D1 level of mercury: Transition probabilities and collisional excitation transfer

Volume 25, number 2

OPTICS COMMUNICATIONS

May 1978

LASER EXCITATION OF 6s6d 63D l LEVEL OF MERCURY: TRANSITION PROBABILITIES AND COLLISIONAL EXCITATION TRANSFER M. ~UKASZEWSKI

and D. LECLER

Laboratoire de Spdctroscopie Atomique, Universitd de Caen, 14032 Caen Cedex, France

Received 11 January 1978 Strong resonance lamp and C.W. tunable dye laser are used for stepwise excitation of 6s6d 63D 1 level of mercury, tile dye laser being tuned to the weak 61P1 ~ 63D1 ?, 5789.6 A line. Atomic transition probabilities for two weak lines starting from the level 63D1 (including the line used for laser excitation) are measured, and the cross sections for the transfer of population from 63Dr level to the other 6s6d levels of mercury induced in collisions with helium atoms are determined.

1. Introduction Efficient selective stepwise excitation of 61 D 2 and 63D 2 levels of mercury using mercury resonance lamp in the first step (61S0 -~ 61P1, k 1850 A) and c.w. tunable dye laser in the second one (61P1 ~ 61D 2 or 63D2, X 5790.6 and 5770 A respectively) was demonstrated by kecler and Laniepce [1 ], and was used to study the population [1] and coherence [2] transfer between 6s6d levels of mercury induced in collisions with noble-gas atoms. The possibility of laser excitation of 63D1 level (61P1 ~ 63D1, X 5789.6 A) was not exploited because of very weak transition probability of the 5789.6 A line. Recently Barrat-Rambosson and Kucal [3) have shown that it is possible to excite stepwisely 6OD1 level via triplet 6s6d levels using a frequency-doubled output of pulsed flashlamp pumped dye laser in the second step of excitation. In the present paper we report an efficient stepwise excitation of 63D 1 level via 61P1 level, i.e. by the weak 5789.6 A line, which has become possible due to several improvements in efficiency and stability of our laser system. Two kinds of measurements have been performed. On the one hand we have measured the cross sections for excitation (population) transfer from 63D 1 level to the other 6s6d levels of mercury induced in collisions with helium atoms in order to complete the ex-

perimental data collected by Lecler and Laniepce [1 ] in the case of excitation of 61D 2 and 63D 2 levels. On the other hand we have determined the relative atomic transition probabilities for two very weak lines starting from the 63D 1 level of mercury, 63D 1 - 63P2 X 3662.9 A, and 63D 1 61P1 X 5789.6 A. These lines had been known to be very weak ones, but no experimental information on their transition probabilities had been available up to the present.

2. Experimental We have used the same experimental set-up which is described in detail in ref. [2], and we have introduced only slight modifications in order to observe fluorescence signals proportional to the populations of the levels. The rotating birefringent plate in the laser beam is replaced by an ordinary chopper modulating the intensity of the beam at the frequency of 813 Hz, and allowing for the lock-in detection. Linear polarization of the laser beam is turned by means of a half-wave plate to be at 54 ° to the direction of the magnetic field in order not to excite the longitudinal components of alignment. The transversal components of alignment are destroyed by the application of sufficiently strong (70 - 80 G) static magnetic field. No polarizers are used in the detection beams.

* Permanent address: Institute of Physics of the Polish Academy of Sciences, 02-668 Warszawa, Poland. 189

OPTICS COMMUNICATIONS

Volume 25, n u m b e r 2 3. T r a n s i t i o n

of 61D 2 level. In this way we obtain the double ratio of atomic transition probabilities as follows:

probabilities

In the experimental conditions described above the intensity of modulated fluorescence signal is proportional to the population of the level:

~(X) = % A(X)S(X), where I~(X) is the fluorescence signal from the level a observed on the line X, na is the population of the level a, A(X) is atomic transition probability for the line X, and S(X) is the overall sensitivity o f the detection system at this wavelength. Measuring the intensities of dit2 ferent lines starting from the same excited level one can eliminate the population of the level, and measure the ratio of atomic transition probabilities of the lines, provided that the sensitivity S0Q can be also eliminated. For that reason we have also excited the level 6 1 D 2 which is very close to the level 63D1, and we have determined the ratio of intensities of the lines starting from the level 61D 2 and terminating at the same 6s6p levels as the studied lines from the level 63D 1 . The corresponding wavelengths being very close (cf. fig. 1), the sensitivities S(X) are the same for the pairs of lines. In the case o f 3662.9 )~ 63D l -- 63p 2 line we have measured the intensities of the lines 3662.9 A and 3131.6 )~ for the excitation of 63D1 level, and the ones of the lines 3663.3 A and 3131.8 A for the excitation

61D"

63D'

6aDa -"I---

6~D~ --7---

3125

3650

6s6d

5789.6 lYE LASER

r

,

_t

'-

I

2~6v

63Po

X \

3663"3X

\\

/

~\ I

_~ 6'P1

6s6p

63P2

1850 MERCURY LAMP

61So-Fig. l. Energy levels and lines of mercury involved in the present experiments. 190

A ( 3 1 3 1 . 6 ) . A(3663.3) _ 3.9(3). A(3662.9) A(3131.8) Absolute value of the transition probability of the line 3662.9 )~ can be obtained by combining the above result with the ones of absorption measurements of Jean et al. [4]. Taking from [4] A(3131.6)/A(3131.8) = 1 andA(3663.3) = 0.14 X 108 s-1, one gets A(3662.9) = 0.036(9) X 108 s-1 . In the case of 5789.6 A 63D1 - 61Pl line the measurements are much more difficult, because it is impossible to avoid a large background due to stray laser light. We have registered the intensity of 5789.6 A line by modulating the laser beam, and looking at the difference between the signals with the mercury lamp on and off or the laser tuned and detuned, and, alternatively, by modulating the mercury lamp, and looking at the difference between the signals with the laser on and off. In all cases the signal to noise ratio is poor, so we are able to give only the estimation of the ratio of transition probabilities: A(5790.6) A (5789.6-) = 32(15). UsingA(5790.6) -- 0.20 X 108 s-1 from [4], we get A(5789.6) = 0.006(3) X 108 s-l .

5790,6

6P,

May 1978

Semi-theoretical calculation of atomic transition probabilities for the 6s6d-6s6p transitions is given in ref. [4]. The wave functions of the levels 6s6d and 6s6p in the intermediate coupling were employed, and the absolute values of the transition probabilities were determined using experimental values of the lifetimes given by L4cluse [5 ]. The comparison of the results of this calculation and our experimental values is given in table 1. One can see that the agreement is very good. It should be remembered, however, that the accuracy of both experimental and theoretical results is not very high $. $ It is to be noticed that in ref. [4] a discrepancy is found between the experimental and theoretical total transition probability for the 63D1 level. The two transition probabilities measured in the present work are so weak that they do not contribute to reduce this difference.

Volume 25, number 2

OPTICS COMMUNICATIONS

Table 1 Experimental values of atomic transition probabilities for the 63D1-63p2 h 3662.9 A and 63D1-6~P1 X 5789.6 A lines of mercury obtained in the present work and their semi-theoretical estimation of Jean et al. [4] Transition

Aexp (in 108 s"q) present work

Ath (in 10 s S t) Jean et al. [4]

63D 1-63 P2 3662.9 A

0.036(9)

0.035

63D1-61 P1 5789.6 A

0.006(3)

0.004

4. Collisional excitation transfer In order to study the collisionally induced excitation transfer from 63D 1 to the other 6s6d levels we measure the intensity of the fluorescence signal from the laser excited level 63D l (direct fluorescence) and the ones from the collisionally populated 6s6d levels (sensitized fluorescence) as a function of foreign-gas pressure. The method of detem~ination o f the population transfer cross section from the measured signals is discussed in detail in ref. [1] (cf. also [2]). Helium at the pressures of 0.1 - 0.5 torr is used as the foreign gas. It is found that for the transfers 63D 1 ~ 63D 2 and 63D 1 -~ 63D 3 this range o f pressure corresponds to the weak pressure region, i.e. that the dependence T/F = f(p) (T and F denote the signals observed in sensitized and direct fluorescence respectively, p is the pressure of foreign gas) can be described sufficiently well by a linear dependence. For the transfer 63D 1 ~ 61D2, which is much stronger than the two previous ones, a non-negligeable curvature of the dependence T/F = f ( p ) is observed (fig. 2). In this case we fit (as in ref. [1] ) a parabola to the experimental points, and we determine the population transfer cross section from its slope at zero helium pressure. In table 2 we give the values o f the population transfer cross sections obtained in the present work, and we compare them with those deduced from the results o f Lecler and Laniepce [1 ] by applying the principle o f detailed balancing. For the transfer 63D 1 -~ 63D 2 the agreement is very good. For the transfer 63D 1 ~ 61D 2 the difference between the two values is quite large, the xatio o f cross sections for the transfers 63D 1 61D 2 (present work) and 61D 2 -+ 63D 1 [1] being

May 1978

/

POPULATION3D~'D2 (3,3,.e/3~3ts)HELIUM _z F 0.4

0.05 /

0

I 03

I 0.2

I 0.3

I 0,4 0.5 p(torr)

Fig. 2. Ratio of signals observed in sensitized (T) and direct (F) fluorescence versus helium pressure (p) for the case of 63D1 --* 6~D2 excitation transfer. Full line is a parabola fitted to the experimental points. smaller than the one given by the principle of detailed balancing. Similar effect was observed in [11 for the transfers 63D 2 ~ 61D 2 and 61D 2 --r 63D2. In both cases one finds the ratios Oa~b/Ob___,a systematically smaller than predicted by the principle of detailed balancing, if b is the level 61D 2. These ratios are proportional to the square o f the ratio of atomic transition probabilities o f the lines used to monitor direct and sensitized fluorescence, and to the ratio of the lifetimes of the levels a and b (cf. [1 ] and [2] ). In consequence the Table 2 Cross sections for the transfer of population betwcen 63Dr level and other 6s6d levels of mercury induced in collisions with helium atoms measured in the present work and deduced from the results of Lecler and Laniepce [11 by applying the principle of detailed balancing oP°P (m A 2) Transfer

present work

Experimental data of ref. [1] +principle of detailed balancing

63Dr -~ 61D2 63D1 ~ 6aD2 63D1 ~ 63D3

66(11) 14.2(1.8) 7.0(1.5)

98(20) 14(3)

191

Volume 25, number 2

OPTICS COMMUNICATIONS

observed deviations from the principle of detailed balancing could arise from the systematic errors in these quantities, in particular in the ones related to the level 61D 2 . We have remeasured the lifetimes of the levels 61 D2, 63D 1 and 63D 2 by the Hanle method. We have used the experimental set-up described in [2], the magnetic field being calibrated with the proton magnetic resonance. Power broadening of Hanle-effect curves [6] has been clearly observed. The lifetimes determined from the zero laser beam intensity limits agree with the resuits of Ldcluse [5], used in [1] and in the present work to evaluate the transfer cross sections, within the experimental errors. In consequence the deviations from the principle of detailed balancing obtained in [1 ] and in the present work cannot be attributed to the errors in the lifetimes of 6s6d levels. We plan to remeasure also the corresponding transition probabilities in order to explain these deviations. In ref. [1] is was suggested that the cross sections for collisional excitation transfer 63D1 -+ 61 D 3 and 61D 2 ~ 63D3 should be virtually the same because o f very small energy difference between the levels 63D1 and 61D2 . This assumption is confirmed by our measurements: the cross section for 63D1 -+ 63D 3 population transfer determined in the present work is equal to 7.0(1.5) A 2, while the one for 61D2 -+ 63D3 population transfer measured in [1 ] is 10(2) A 2.

5. Conclusion Despite the very small transition probability for the

192

May 1978

6 1 P 1 - 6 3 D I X 5789.6 Aline of mercury we have obtained the population of 63D 1 level sufficiently large to observe quite well measurable fluorescence signals. Although the signal to noise ratio is not sufficiently good to permit very precise measurements, it allows nevertheless to get a valuable spectroscopic information. We have been able to measure, or at least estimate, the atomic transition3 probabilities for two weak lines of mer~ cury, 6 3 D 1 - 6 P2 X 3662.9 A, and 63D1-61P1 ?, 5789.6 A, the experimental results being in very good agreement with the theoretical predictions. It has been also possible to study the transfer of population from 63D1 level to the other 6s6d levels of mercury induced in collisions with helium atoms, despite that the signals in sensitized fluorescence are again several times smaller than the ones in direct fluorescence. In this way we have been able to complete the experimental data on collisional excitation transfer between 6d6d levels of mercury obtained previously by the other, stronger, excitation schemes.

References [1 ] D. Lecler and B. Laniepce, J. Physique 37 (1976) 55; D. Lecler, B. Laniepce and F. Sage, J. Physique 37 (1976) 1173. [2] M. Lukaszewski and D. Lecler, J. Phys. B: Atom. Molec. Phys., to be published. [3] M. Barrat-Rambosson and H. Kucal, private communication. [4] P. Jean, M. Martin, J.P. Barrat and .I.L. Cojan, C.R. Acad. Sci. 264 B (1967) 1709. [5] Y. Lecluse, J. Physique 28 (1967) 671. [6] M. Ducloy, Phys. Rev. A8 (1973) 1844; 9 (1974) 1319; Th~se Paris (1973).