Xe2Cl and Kr2F excited state (4 2Γ) absorption spectra: measurements of absolute cross sections

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CHEMICAL PHYSICS LETTERS

Xe,Cl AND Kr2F EXCITED STATE (4 ‘r) ABSORPTION MEASUREMENTS OF ABSOLUTE CROSS SECTIONS

SPECTRAz

D.B. GEOHEGAN ’ and J.G. EDEN Department of Electrical and Computer Engineering,

University ofIllinois, Urbana, IL 61801, USA

Received 15 August 1986; in final form 27 April 1987

Absolute absorption cross sections for the lowest excited states (4 ‘I ) of Xe,Cl and Rr2F have been measured at discrete wavelengths in the visible and ultraviolet ( Xe,Cl: 325 6 I < 495 nm; KrZF: 248 Q I Q 570 nm) The wavelengths observed for the 9 ‘I +4 ‘I band peak in each spectrum confirm the predictions of theory. However, the spectral profile and magnitude of the Xe,CI and Kr2F absorption cross sections are considerably different from those predicted theoretically for the corresponding rare gas dimer ions, Xe$ and Kr,+ .

1. Introduction

Discovered in 1976, the rare gas halide triatomic molecules (Rg,X; Rg and X are rare gas and halogen atoms, respectively) exhibit several interesting chemical and optical properties #’ [ 2-41. Although their ground states are dissociative, the lowest excited state (4 2F ) is bound and believed to be ionic in nature (Rg: X- ). An isosceles triangle configuration for the molecule is expected to be the most stable [ 21 as it best preserves the resonance interaction energy ( x 1 eV) that binds the rare gas dimer ion, Rg: . That is, theory predicts that an approach of the halogen anion along a line that bisects (and is normal to) the Rg: internuclear axis will have minimal impact on the RgRg+-Rg+Rg resonance stabilization [ 2-41. According to this argument, the Rg2X( 4 *F ) and Rg$ 1( f )” dissociation energies and absorption cross sections should be nearly identical

[VSI. The results of preliminary measurements of the absolute absorption cross sections (a) for the 4 2F excited states of Xe2C1 and Kr2F in the visible and ’ Visiting Research Associate: 1986-87; present address: Solid State Division, Building 2000, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37831-6056, USA. iii For a review of the chemical and optical properties of these molecules, see ref. [ 11.

0 009-2614/87/$

(North-Holland

near-ultraviolet (UV) are reported in this Letter #2. Prompted by a desire to extend previous measurements into the visible, they confirm earlier experimental indications [ 8,9] that the Rg2X excited state absorption cross sections (associated with the 9 ‘F t4 2F band) are significantly smaller than those for the analogous transition in the dimer ion ( 2 ( 1)9+- 1 ( 4 ) . ) . However, the absorption spectral profiles for the trimers verify theoretical predictions regarding the position of the 9 2F t4 ‘F band peak. Also, evidence of the 82F +-4’F transition is observed for both Xe2C1 and Kr2F in the blue (1~435 and 472 nm, respectively). McCown et al. [ 91 have previously demonstrated the ability to selectively observe Xe2Cl( 4 *F ) absorption but their measurements were confined to several wavelengths between 193 and 351 nm.

2. Experimental

approach and apparatus

The general experimental approach followed in these studies has been described previously [ 91. Briefly, the initial experimental step is to produce the 42F excited state of Kr2F or Xe2Cl by illumi” The Xe,Cl results were first described in ref. [ 61. The Rr2F data were recently described in ref. [ 71.

03.50 0 Elsevier Science Publishers B.V. Physics Publishing Division)

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nating IWF, or Xc/Cl, gas mixtures, respectively, with the focused beam from an excimer laser. In the K.r*F experiments, formation of the excited triatomic is initiated by photoionizing atomic Kr (in a 1000 Torr IWl.0 Torr Fz gas mixture) at 193 nm ( Aw = 6.4 eV, ArF laser) by a three-photon process [ lo], resonantly enhanced by the Kr 6p [ $lZ state at 103363.42 cm- ‘. This 2+ 1 multiphoton ionization process produces Kr* excited species (6p, 4d, 5p, 5s’, 5s) as well as RI-+(*P,,*, 3,2) ions, both of which are subsequently involved in the formation of KrF*( B,C) by harpoon collisions (Kr* (or Krf) + F2+KrF* + F) or by three-body ion-ion recombination(Kr+ (orKr:)+F-+Kr-+Kr~+ISr).The Kr2F molecule is then rapidly produced (r x 1.5 ns for PKr= 1000 Torr) in its lowest ion pair excited state, 4 *I, by three-body collisions of the diatomic excimer with two background Kr( ‘So) atoms. An alternate method of populating Kr2F( 4 *I ) was used for several of the photoabsorption measurements. KrF(B) molecules were formed directly by photoassociation (bound t free absorption) - that is, the absorption of a 5 eV photon (2 = 248 nm, KrF laser) by a colliding Kr-F pair - and subsequently converted to Kr2F by a three-body process. The Kr-F photoassociation process, including its wavelength dependence, will be described in detail elsewhere [ 111. All of the Xe2Cl experiments were carried out by first creating XeCl(B) molecules by photoassociation of Xe-Cl pairs with a 308 nm laser [ 121. For these latter studies, the gas mixtures in the optical cell were typically 300 Torr Xe/O.5 Torr Cl*. At an adjustable time delay At following the initial, excimer laser pulse, the beam from a pulsed dye laser ( x 15 ns pulse width, ~0.2 cm-’ linewidth) was directed along the axis of the cylindrical quartz cell containing the gas mixture. In an effort to match the cross-sectional area of the dye laser beam to the width of the ArF beam at its focus, the dye beam was spatially filtered by an aperture and then compressed in one dimension by a cylindrical lens. Consequently, the dye laser beam illuminated only the region lying along the focus of the excimer laser (coinciding with the axis of the cell) where the Rg2X* number density was maximum. Visible Rg,X fluorescence emanating from the cell was viewed at 90” to the dye laser beam axis with a rectangular slit/optical telescope apparatus that rejected sponta520

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neous emission originating from regions of the cell other than at the intersection of the two laser beams. The absorption of a dye laser photon by the triatomic species in this region was detected as a tranof the Rg2X(4*I+1 *I) sient suppression spontaneous emission in the visible (Xe&l: Ax 490 nm; Kr2F: 1 z 400 nm). It should be noted that the use of a dye laser to photoexcite the Rg,X( 4 *r ) species - rather than employing the scheme [ 91 of two counterpropagating excimer laser beams - increases the difficulty of the experiments. Not only is the alignment of the dye and excimer beams more critical but the leakage of off-axis fluorescence through the spatial filter detection apparatus leads to a measured suppression of the collected fluorescence which is smaller than its true value. However, the advantage of illuminating only the ArF focal region with dye laser radiation is that scaling of the measured absorption (due to a spatially non-uniform Rg,X* density profile) [ 91 is no longer required. As a check on the experimental approach, additional measurements of the Kr2F( 4 *I ) absorption cross section were made at three wavelengths in the UV (248, 308 and 351 nm) in which the timedelayed probe pulse was provided by an excimer laser. For these experiments, the rectangular excimer beam was collimated and compressed in both dimensions by two pairs of orthogonally oriented cylindrical lenses and, as in the dye laser studies, directed along the focus of the initial (ArF) laser beam. As will be shown later, both the Xe2Cl and Kr2F absorption cross sections measured at I= 35 1 nm with a dye laser agree to within experimental error with the values obtained with an excimer laser (XeF) at the same wavelength. The interpretation of the Rg2X fluorescence suppression (by the second laser pulse) in terms of an absolute cross section has been discussed previously [ 9,131. Given an absorptive transition having a cross section 0, the magnitude of the fractional suppression, FS (FS= 1 corresponds to total suppression), of the fluorescence varies with the dye laser fluence @ as FS cc 1- exp( -a@). Knowledge of the absolute absorber number density is, therefore, not necessary in order to determine cr, assuming that one has sufficient dye laser fluence to saturate the exponential (i.e. FS E 1-e- ’ ). Simply fitting the experimental FS versus 0 curve to the expression

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given above yields the absorption cross section. The fluence, Cp= (,WzoA ) , is calculated from the measured dye laser pulse energy E and the cross-sectional area of the beam, A. A forthcoming publication [ 141 will discuss the details of the experimental procedure and data analysis. For each dye laser wavelength studied, over 1000 suppression points (FS, E ordered pairs) were measured and stored on a computer. Subsequently, 0 was

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Fig. 1. Fractional suppression (FS) of KrzF (4 *F ) fluorescence (expressed in %) induced by a dye laser pulse of I = 400 nm. The top panel shows the results of 1000 measurements of FS, @ (or dye laser energy, E) ordered pairs. The lower portion of the figure illustrates the least squares fit of the equation: FS=a[ 1 -exp( -bE)] (indicated by the solid curve) to the raw data. Also shown in (b) is the set of 50 points resulting from averaging the data in (a). The best Iit of the FS versus E curve uniquely determines b = a( frwA)- ’ and, hence, u. The estimated uncertainty in the cross section measured at this wavelength is zk5%.

11 September 1987

determined from the least squares tit of the expression FS= FS,,, [ 1-exp( -a@)] to the data. Representative data are shown in fig. 1 for Kr2F( 4 *I ) absorption at 400 nm. The results of measurements of Kr2F fluorescence suppression for various values of dye laser energy, E (or fluence Q), are given in the top panel. As illustrated in the bottom half of the figure, after averaging the raw data points, u was determined from a least-squares fit of the expression given above to the raw (and averaged) data to be 2.4x lo-‘* cm2. Deviations between the best fit (indicated by the solid curve in fig. 1) and the raw data points were < f loo/6 while the difference between the D determined from the least-squares fits of the FS( @, a) equation to the raw and averaged data were typically less than 2%. The primary source of error in the cross-sectional measurements lies in the accurate determination of the dye laser beam area. Since the lit to the measured FS versus E points determines the ratio a/A, and the parameters FS, E and Adyeare accurately known, it is clear that essentially all of the error in the calculation of d arises from the uncertainty in the measurement of A. The difficulty stems from the non-uniform crosssectional intensity profile for the dye laser beam. In order to determine the isointensity profiles for the dye beam, a set of bum patterns was made on unexposed, uncoated Polaroid film by systematically varying the number of shots and the energy in the beam. These measurements were repeated for each of the seven dyes and the three excimer wavelengths involved in the experiments. The beam area for a specified number of laser pulses and fixed beam energy was determined with a calibrated optical microscope. Plotting the measured beam areas as a function of pulse energy and number of laser shots then made it possible to determine the isointensity contours for the beam. The cross-sectional area for each dye was chosen to be that which contained z 85% of the total beam energy. This determination was made for each dye at a pulse energy of 1.2 mJ. It is estimated that the measurement of the dye laser beam area introduces a (dye-dependent) uncertainty (that is constant throughout the scanning region for a dye) of < + 30% into the cross sections reported here. 521

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3.Results and discussion Kr,F

The spectral variation of the Xe&l and KrzF 4 2r absorption cross sections in the visible and UV (Xe2Cl: 325 ~1~495 nm; Kr,F: 248~A~570 nm) is shown in figs. 2 and 3, respectively. Both spectra are dominated by maxima at x335 nm which are attributed to 9 ‘T t4 21Ytransitions of the triatomic species. The experimentally observed wavelengths of the band maxima correspond closely with the predictions of theory which links the 9 2r ~4 ‘T band of Rg$ X- with the 2( f ),- 1( 4)” transition of the dimer ion. Wadt and Hay [ 21 calculated the peak of the 9 ‘T t4 ‘I’ absorption band of Kr2F to lie at 336 nm while the analogous transition for Xe,Cl was predicted by Stevens and Krauss [ 51 to occur at 339 nm. Note also the similarities in the peak cross sections (omax = 1 x lo-” cm*) and the long wavelength segments (A> 350 nm) of the spectral profiles for both molecules. When compared to the theoretically predicted Kr$ and Xe: absorption spectra [ 15 1, however, the Xe2Cl and Kr2F data of figs. 2 and 3, respectively, exhibit significant absorption further to the red than expected. Furthermore, the latter observation cannot be explained by comparing our triatomic absorption data with the UV absorption spectra for the hot ( T> 300 K) Rg: dimer ion [ 41.

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Fig. 2. Absorption spectrum of XezCl (4 2F ) in the visible and near-UV ( 325 < 1 Q 495 nm) . For convenience, the cross sections reported in ref. [ 91 for several wavelengths between 193 and 35 1 nm are also shown (represented by A ). The uncertainty in c for each datum is indicated and is 5 f 30% for all of the points. The gas mixture for these data was 300 Torr Xe, 0.5 Torr Cl,.

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Fig. 3. KrzF (4 *I ) absorption cross section lengths between 248 and 570 nm. Again, the tainty in u for each point is 5 ?30%. Those acquired with two overlapped excimer laser denoted by solid triangles ( A ).

at various waveestimated uncerdata which were beams are again

Although the higher temperature Rg: spectra of ref. [ 41 are indeed noticeably broadened to the red, they nevertheless remain symmetric around the peak wavelength ( % 340 nm) whereas the Kr2F and Xe2C1 data reported here are clearly skewed towards the red. For Xe2C1, the peak cross section in fig. 2 (9.3 x 1O-‘8 cm’) is within 10% of that reported in ref. [ 91 (for A= 337 nm) despite the differences in beam geometries (and the numerical scaling factors) in the two experiments. However, this value for cr is z l/4 of the Xe$ {2( f ),t 1( +)U} photodissociation cross sections measured by Vanderhoff [ 161 and Lee and Smith [ 171 (3.8x10-” and 2.96x10-” cm2, respectively) for fiw z 3.5 eV. The peak Xe,Cl cross section is also smaller (by= 40°h) than the Xe: cross section measured by McCown et al. [ 131 at ,I = 337 nm (1.44x lo- ” cm*) but, to within the combined experimental errors, the two results are in agreement. The maximum value of the Kr2F absorption cross section in fig. 3 is 1.0x 10-l’ cm* (at A=337 nm) but, at 360 nm, o has fallen to z 3 x lo-‘* cm2. The latter is less than l/3 of the Kr: photodissociation cross section that has been measured at 362 nm in preliminary experiments conducted in this laboratory and is much smaller than the value ( x 2 x 1O- ” cm*) obtained when Wadt’s [ 151 calculated Kr: absorption profile (300 K) is scaled to the data of Lee and Smith [ 171 and Vanderhoff [ 161. Remarkably, the cross section measured at 358 nm

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((5.0+1.7)x10-‘*cm2),is, to withinexperimental error, identical to that estimated by Eden, Chang and Palumbo [ 81 for Kr,F(4 2F ) in electron beam experiments. Therefore, the present experiments appear to confirm the trimer absorption cross sections reported in the only two previous measurements in the literature [ 8,9]. The other most conspicuous aspect of the Xe,Cl and Kr2F spectra in figs. 2 and 3 is the presence of secondary maxima. Theoretically speaking, the only other predicted transition lying within the experimentally investigated spectral range is the 8 2F +-4 2F band of the trimers. For Xe2C1, Stevens and Krauss [ 51 predicted such a feature (at 438 nm) which coincides with a local maximum at 1% 435 nm in fig. 2. The 8 2F t4 2F transition of Kr2F, calculated by Wadt and Hay [ 21 to occur at 1= 478 nm, also nearly matches a local maximum of Kr2F in fig. 3 at Ix 472 nm. In both cases, however, the measured cross sections are considerably stronger than would be expected on the basis of the relative 9 ‘F ~4 21’ and 8 ‘F ~4 ‘F oscillator strengths calculated in ref. [ 51 (Xe,Cl: 0.3 and 3.0x 10-4, respectively) and ref. [2] (Kr,F: 0.41 and 5.2 x 10e4, respectively). The Xe,Cl and Kr2F absorption profiles differ markedly for A<340 nm. Between 2~337 and 308 nm, the Kr2F cross section falls by a factor of z 5 which is a more abrupt decline than that calculated [ 151 for the Kr$2( f ),+- l(f)” transition over the same spectral region. Xe2C1excited state absorption, on the other hand, remains strong and nearly constant for wavelengths as short as 193 nm which may be attributable to photoionization of the excited trimer [9]. The absorption cross section for Kr2F at 248 nm (0=(1.6+0.8)x10-‘~ cm2) should be useful in assessing the impact of triatomic rare gas halide excited state absorption on the KrF laser. The measurements at this wavelength were complicated by the generation of Kr2F( 4 2F ) fluorescence by the 248 nm probe laser due to production of KrF(B) by photoassociation. In determining the 248 nm absorption cross section reported above, the photoassociation process was accounted for by compensating for the Kr,F emission produced by the second (time-delayed probe) laser pulse and by minimizing the free fluorine formed by F2 photodissociation as a result of the first excimer laser beam. Details of the analysis

11 September 1987

that is the basis for data reduction and the results of a study on the photoassociation process itself will be presented elsewhere [ 11,141. Finally, it should be noted that the 4 2F states of Xe2Cl and Kr2F are clearly absorbing strongly in the blue and blue-green where lasing has been observed on the 4 2F -+ 1 2F transitions (Xe,Cl: AZ 520 nm [ 181; Kr2F: 1x430 nm [ 191, 450+ 10 nm [20]). This situation not only has an obvious and adverse impact on the extraction efficiency of such lasers, but confirms the cause for the observed red shift of the laser spectrum with respect to the spontaneous emission band. In summary, preliminary measurements of Xe2C1 and Kr2F excited state (4 ‘F ) absorption cross sections in the visible and UV have been reported. Despite the presence in the literature of only a few measurements of Rg2X cross sections, all consistently find that a(Rg: X-)
Acknowledgement

The authors would like to express their appreciation to Dr. A.W. McCown, J.H. Schloss and V. Tavitian for their assistance and many useful discussions. Also, the excellent technical assistance of D. Banks, IS. Kuehl and Y. Moroz is gratefully acknowledged. This work was supported by the Office of Naval Research (R. Behringer, V. Smiley) under contract NOOO14-85-K-0739 and the Los Alamos National Laboratory under contract LANL 9X65W1489.

References [ 1] D.L. Huestis, G. Marowsky and F.K. Tittel, in: Excimer lasers, 2nd Ed., ed. C.K. Rhodes (Springer, Berlin, 1984) pp. 181-215. [2] W.R. Wadt and P.J. Hay, J. Chem. Phys. 68 (1978) 3850, and references therein. [ 31 D.L. Huestis and N.E. Schlotter, J. Chem. Phys. 69 (1978) 3100. [4] H.H. Michels, R.H. Hobbs and L.A. Wright, Chem. Phys. Letters 48 (1977) 158. [ 5 ] W.J. Stevens and M. Krauss, Appl. Phys. Letters 4 1 (1982) 301.

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[6] A.W. McCown, D.B. Geohegan and J.G. Eden, Visible and UV absorption spectrum of Xe&l, paper THW6, Conference on Lasers and Electra-Optics (CLEO) 1985, Baltimore, MD (May, 1985). [7] D.B. Geohegan and J.G. Eden, Visible and ultraviolet absorption spectrum of Kr,F( 4 ‘F ), paper FF12, International Laser Science Conference (ILS) II, Seattle, WA (October, 1986). [8] J.G. Eden, R.S.F. Chang and L.J. Palumbo, IEEE J. Quantum Electron. QE-15 (1979) 1146. [ 91 A.W. McCown, M.N. Ediger, D.B. Geohegan and J.G. Eden, J. Chem. Phys. 82 (1985) 4862. [ lo] D.B. Geohegan, A.W. McCown and J.G. Eden, Phys. Rev. A33 (1986) 269. [ 111 J.H. Schloss, D.B. Geohegan and J.G. Eden, unpublished.

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[ 121 A.W. McCown and J.G. Eden, J. Chem. Phys. 81 (1984) 2933. [ 131 A.W. McCown, M.N. Ediger and J.G. Eden, Phys. Rev. A28 (1983) 3365. [ 141 D.B. Geohegan and J.G. Eden, unpublished. [ 151 W.R. Wadt, J. Chem. Phys. 68 (1978) 402; 73 (1980) 3915. [ 161 J.A. Vanderhoff, J. Chem. Phys. 68 (1978) 3311. [ 171 L.C. Lee and G.P. Smith, Phys. Rev. Al9 (1979) 2329. [ 181 F.K. Tittel, W.L. Wilson, R.E. Stickel, G. Marowsky and W.E. Ernst, Appl. Phys. Letters 36 (I 980) 405. [ 191 F.K. Tittel, M. Smayling, W.L. Wilson and G. Marowsky, Appl. Phys. Letters 37 (1980) 862. [20] N.G.Basov,V.S.Zuev,A.V.Kanaev,L.D.MikheevandD.B. Stavrovskii, Kvantovaya Elektron. (Moscow) 7 ( 1980) 2660 [English transl. Soviet J. Quantum Electron. 10 (1980) 15611.