Volume 156, number 6 CHEMICAL PHYSICS LETTERS 2 I April 1989

RADIATIVE LIFETIME OF THE Tel A(Oi) STATE STUDIED BY LASER-INDUCED FLUORESCENCE

E. MARTfNEZ, P. PUYUELO, F.J. BASTERRECHEA and M.T. MARTfNEZ Departamenro de Quimica Firica, Facultad de Ciencias, hive&dad del Pais Vasco, Apartado 644. 48080 Bilbao. Spain

Received 27 May 1988; in final form 24 January 1989

Time-resolved laser-induced fluorescence of the A(O: ) state of natural tellurium dimers has been observed. Excitation to vibrational levels 5 < U’ 4 10 has been carried out by scanning the wavelength of a Nd-Yag-pumped narrow-band dye laser from 503 to 520 nm. The radiative lifetime of the A state of Tc, (5 G u’ & 10) was determined to be IOROk60 ns. A collision cross

section of u= 139 k 5 A’ was obtained for deactivation ofthe A(O: ) state.

1. Introduction

The homonuclear diatomic molecules of the group VI elements OX, S2, Se2 and Te2 form an interesting series for spectroscopic study and investigation of the excited state dynamics. However, while information on the spectroscopy and dynamics of the excited states of O2 and S2 is quite extensive [ 11, limited in- formation is available on Se2 and Te,. With increas- ing mass the molecular spectra generally become more complex.

In O2 and S?, the ground electronic state is 3Z; ; however for the heavier diatomic molecules spin-or- bit coupling becomes more important and a Hund’s case “c” notation should be used. With increasing spin-orbit coupling the ground state splits into a (0: ) ground state, and a lg state higher in energy. A similar splitting will occur in the higher states. The major bands involved in the visible and near ultra- violet absorption spectra of Tez are the A (0: ) and B(O:)-X(0.$) band systems [2-51. The study of the spectroscopy and dynamics of the A(Oz) and B( 0: ) excited states of Te2 is of interest for several reasons. First, the spin-orbit effect ought to be the largest in the series, showing a purest case “c” be- haviour. Second, optically pumped continuous laser emissions of Tez molecules in the visible [ 7,8] and near infrared [ 81 spectral range have been reported.

Laser-induced fluorescence (LIF) provides a nat- ural way of studying the dynamics of quantum-se-

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lected states of electronically excited molecules. Pulsed excitation enables one to measure the fluo- rescence decay in real time leading to rate constants for both collision-free and collisional (electronic quenching and ro-vibrational transfer) processes. The aim of the present work is to obtain the zero-pres- sure lifetimes and collision cross sections of the A( 0,’ ) state of Tez. Knowledge of both values is of interest, mainly in investigating potential laser pos- sibilities of this molecule.

2. Experimental

The experimental arrangement was substantially the same as described previously [ 9, lo]. In the pres- ent experiment, tellurium dimers were produced by heating solid pure tellurium powder (99.999% Gold label Aldrich Chemical Co.) in a cylindrical Pyrex cell in a home made oven at 600-740 K. The cell was connected to a vacuum system for purification and outgassing, since systematically shorter lifetimes were obtained before these operations. A useful range of Tel(g) pressures, between 3 and 500 mTorr, could be obtained by controlling the temperature in the cell

1111. Laser excitation spectra were recorded by moni-

toring the undispersed fluorescence intensity as a function of laser wavelength. The third harmonic ( z 355 nm ) of a Nd-YAG Quantel laser, delivering

Volume 156, number 6 CHEMICAL PHYSICS LETTERS 21 April 1989

pulses of 85-90 mJ, was used to pump a Quantel dye laser. Methanolic solutions of coumarin 500 were used to produce laser pulses of l-2 mJ/pulse, be- tween 500 and 520 nm. The pulse width was about 15 ns fwhm, and the laser linewidth was less than 0.15 cm-‘.

Laser-induced spectra were recorded by detecting induced fluorescence by means of a high-gain, fast photomultiplier tube EM1 98 16B and using a boxcar averager (Brookdeal 9415/9425) as a function of laser wavelength. Cut-off filters Schott OG530 were used to filter out scattered laser radiation, as well as an Oriel 58883 interference filter, to block red emis- sion from the oven resistor. A Tektronix 2430A tran- sient digitizer was used to obtain the fluorescence decays from the photomultiplier output, these sig- nals were averaged and then processed with an In- vestronic PC 640 X system, giving the fluorescence lifetimes through standard statistical programs.

Absolute wavenumber calibration of the dye laser scanning system was achieved by recording the I2

spectrum in this spectral region. I2 lines were as- signed using the I2 Atlas of Gerstenkorn and Luc

[121-

3. Results and discussion

Excitation spectra of TeZ were obtained by scan- Fig. 1 shows a typical decay for excitation at 5 197.8 ning the laser wavelength between 500 and 520 nm, A, and a fast decay in the initial part ofthe curve (up where several rotational transitions of strong bands to 200-300 ns), is readily observed, corresponding

of the A-X system, as well as a few lines of very weak bands of the B-X system are expected. That emis- sion, which occurred essentially from the long-lived A( 0:) state, was checked by obtaining excitation spectra at different integration delays from the dye laser pulse, from 250 up to 1000 ns, where no ap- preciable differences in the relative intensities be- tween the different spectra were observed.

Even though the B-X and A-X band systems of tellurium dimers show simple P and R structures, the low magnitudes of the rotational and vibrational constants, in addition to the large number of isotopic species present in natural tellurium vapor, lead to a dense and overlapped spectrum, and no rovibra- tional assignations were possible for the different isotopic species. Only an approximate vibrational

assignment, within our laser spectral resolution of’ ~0.03 A, was possible under these conditions. Table 1 lists some of the laser excitation wavelengths used to induce fluorescence from the excited states within the spectral range studied. Assignation of the B-X and A-X bands, some of whose rotational transi- tions for the different isotopic species are expected to be excited has been made using the recent data for the A, B and X states of tellurium dimers [ 5,13,14 1. The B-X bands, in this region, are expected to be much weaker than the A-X bands in laser-induced fluorescence experiments.

Table 1 A(O: )-X( 0.C) and B( 0: )-X(0: ) d-u” bands observed in laser-induced fluorescence at selected wavelengths in the interval 503.6- 5 19.5 nm. Expected intensity, I, is expressed in parentheses (w: weak, m: medium. s: strong)

Band system

Excitation interval (A)

5036.6-5037.9 5121.3-5129.9 5152.0-5159.7 5 194.4-5 197.8

A-X 7-2 (m) 6-3 (m) 6-3 (m) 5-3 (w) “‘-0” 8-2 (w) 7-3 (m) 7-3 (w) 6-3 (w)

9-2 (w) 8-3 (w) 7-4 (s) 6-4 (s) 9-3 (s) 9-3 (w) 8-4 (m) 7-4 (m)

IO-3 (m) 8-4 (s) 9-4 (w) 8-4 (w) 9-4 (w) 8-5 (m)

B-X O-9 (m) O-10 (w) O-11 (m) O-12 (m) “’ -&l” O-11 (m)

o-12 (w)

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Volume 156, number 6 CHEMICAL PHYSICS LETTERS

h 1 z

4

to weak emission of the short-lived B state. All curves showed exponential behaviour from 400 ns of the laser spike upwards, characteristic of lifetimes around I FS, for low Ter pressures. Statistical analyses to get the lifetimes were carried out from 400 up to 4000 ns from the laser spike.

Collision-free lifetimes and total quenching colli- sion cross sections were determined by measuring fluorescence lifetimes in the range of Tez pressures from 3 to 500 mTorr. Fig. 2 shows a typical Stern- *

0 Volmer plot for excitation at 5 152.7 A, where in- 0

’ TfME(;ls) 4 5 verse lifetimes are plotted against P/T ‘/2 giving the

collision cross sections through

Fig. 1. Semilogarithmic plot of variation of the fluorescence in- tensity (arbitrary units) for emission from the A(OJ ) state after

excitation at 5 197.8 A, against time. Tel pressure is 5.5 I mTorr.

l/r=1/~~+(8/n~K)“~~P/T”~. (1)

No substantial variation, beyond the experimental

2 1 April 1989

G

h

-i

u)3 .

*s F -

2

I +

I I I I

5 10 15 xl

P/ fl( 10-3.Torr - K-“* ) Fig. 2. Stern-Volmer plot of the inverse lifetime (5) agamst P/T “’ (P in Torr, Tin K) after excitation at 5 I 52.7 A. The pressure range

is 3-496 mTorr.

Volume 156. number 6 CHEMICAL PHYSICS LETTERS 2 I April 1989

Table 2

Summary ofcollision-free lifetimes ( rO) for Tez A(O,+) states at selected excitation wavelengths. The vibrational quantum num-

bers (u’ ) of the emitting levels of the A state are shown; the levels from which emission is most intense arc given in italics

Excitation To”’ line (A) (ns)

5036.6 1007&42 5036.7 1041 f25 5037.6 984? 17 5037.9 1002f29

5124.3 1121&35 5129.5 1082+31 5129.9 1153?24

5152.0 1085 &40 5152.7 1097 * 102 5159.7 107Ok46

5194.4 1132?24 5 195.9 1118+21 5197.8 1188&22

” Two standard deviation errors are given.

v’ (A)

7, 8, 9, 10

7, 8, 9, 10 7, 8, 9, IO 7, 8, 9, 10

67, 8,9

67, 8,9

67, &9

6, 7.8

6, 7,8

6, 7,8

5, 6, 738 5, 6. 7.8

5, 6, 7,8

accuracy, was observed for the collision cross sec- tions. We obtained a mean value of 139 f 5 A’, for laser excitation at 5 129.9, 5 152.7, 5 159.7 and 5 194.4 A. From experimental measurements of 5 (see table 2 ) , andiusing a value of o= 139 & 5 A’, collision-free lifetimes can be obtained through formula ( 1) for the whole set of studied emissions, as shown in table 2.

The collision-free lifetimes (TV) which are con- nected with the decay rate f ( = l/r, 7 being the ex- perimental lifetime) by the relation r=r;’ +r,,,, where r,,, is the collisional deactivation rate, are presented in table 2. As can be seen, laser excitation from 503 to 520 nm produces induced fluorescence from low vibrational levels of the A(O: ) state, showing lifetimes around and above 1 ps at low Tez pressures, and giving collision-free lifetimes slightly higher than 1 us. A mean value of 1.08 i 0.06 ps has been obtained for emission from U’ =5-l 0 levels of the A( 0: ) state, although the lifetime seems to in- crease slightly going down the vibrational manifold. However a more careful study with an unambiguous vibrational assignment of selected isotopic species needs to be done to confirm this apparent tendency. The radiative lifetimes presented here are larger than

those previously reported by Thorpe et al. [ 151 of 730 ns (emission from v’=S, A state) and Ferber et al. [ 161 of 670 ns (from Y’= 11, A state), both in- vestigations only involved a single excitation line. In Thorpe’s work, for emission from the A state, the U’ value is in the same range as in this work, but life- times were obtained in a pressure range above 2 Torr, without obtaining any data from the interesting and definitive low pressure regions, close to collision-free conditions.

Much shorter lifetimes (60-70 ns) have been re-

ported by Cariou et al. [ 171 after excitation to the levels v’ = 16-17 of the A state. However no evi- dence of the nature of the emitting state is given, and in any case a more careful investigation in this spec- tral region (excitation below 480 nm) for isotopi- tally enriched species of Te7 should be done, in order to obtain radiative lifetimes for the B state and pos- sible perturbations of the A state vibrational levels v’ = 10-20, as reported by Cariou et al. [ 171.

Collision cross sections of 139 ?I 5 A’ have been obtained for quenching of the A (0: ) state for v’ = 6- 9 levels below the Te ( ‘PI ) + Te ( 3P2 ) dissociation limit. This value agrees with those reported by Cariou etal. [17] of136+6A’(v’=16, 17),andFerberet al. [16]of160~30A’(v’=1l),forlevelsabovethe Te( ‘Pz) +Te( ‘Pz) dissociation limit.

Acknowledgement

The financial support from Gobierno Vasco and Universidad de1 Pais Vasco is gratefully acknowl- edged. One of us (P.P.) wishes to thank Gobiemo Vasco for a grant.

References

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