J . Phys. Chem. 1990, 94, 8913-8917
8913
Production of Rare Gas-Halide Excimers following Trimer Absorption at Ultraviolet Wavelengths A. W. McCown Los Alamos National Laboratory, Laser Science and Applications Group, Los Alamos, New Mexico 87545 (Received: April 19, 1990)
Absorption by the rare gas-halide trimers Kr2F(42r)and Xe2C1(42r)has been shown to result in the immediate formation of the respective excimer, KrF(B,C) or XeCl(B,C), and the branching ratio for this process has been measured at several ultraviolet wavelengths. In experiments using excimer lasers as both pump and probe, the branching ratio for both trimers has been found to be 0.9 0.1 for wavelengths above 300 nm and approximately zero at wavelengths shorter than 250 nm. These results suggest that the trimers have strongly repulsive potential energy curves that terminate on the excimer levels. This is not predicted by theoretical modeling and demonstrates the need for improvement in the theoretical understanding of the trimers and perhaps even the excimers.
*
I. Introduction The rare gas-halide trimers, and in particular Xe2CI and Kr,F, have received a considerable amount of attention recently. In the past few years, three separate laboratories have reported measuring the Kr2F(42r)absorption cross section at 248 nm,1-3with one of the groups extending the measurements from 335 to 600 nm using a dye laser system.l Prior to these measurements, studies had been made on Xe2CI* absorption in the ultraviolet (UV)4 and Kr2F* absorption in the nea~--UV.~Theoretical analyses had predicted that, a t wavelengths from 250 and 500 nm, absorption should take place from the 421' to the 921' state, leaving the trimer molecule highly excited but bound.6-'0 The trimer absorption spectrum was expected to be similar to the associated dimer ion 1 ( 1 /2)" 2( 1 /2& spectrum, which was predicted to have an -80-nm bandwidth." This paper reports the results of experiments that have shown that the KrF(B,C) and XeCI(B,C) states are created following the photodissociation of the respective trimer at absorption wavelengths greater than 300 nm.I2 The branching ratio for this process has been measured at discrete wavelengths as short as 193 nm for both trimers. Experiments have been performed utilizing optical pumping of the species of interest, allowing direct measurements of absorption, laser-induced excimer fluorescence, and the branching ratio to be made in the absence of competing species. This paper is organized in the following way. Section I1 describes the experimental setup, while section 111 outlines the optical pumping scheme for XeCl and KrF. The manner in which branching ratios were determined is given in section IV, and experimental results and a discussion are presented in section V while the conclusions are summarized in section VI.
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11. Experimental Section The experimental setup has been described in detail elsewhere2q4 and will only be presented briefly here. Two 15-11s full width at half-maximum (fwhm) excimer lasers were aligned so that their ( I ) Geohegan, D. B.; Eden, J. G. J . Chem. Phys. 1988,89, 3410. (2) McCown, A. W. Appl. Phys. Lett. 1987, 50, 804. (3) Hakuta, K.; Komori, H.; Mukai, N.; Takuma, H. J . Appl. Phys. 1987,
beams were counterpropagating and focused to a common line along the axis of a quartz cell by means of cylindrical lenses. Mixtures of Xe and C12 or Kr and F2 were introduced into the cell with a gas handling system and were irradiated by the pump beam, causing optical excitation of the excimers and trimers. The probe beam was then fired after adjustable time delay, resulting in absorption by the trimer. This was evidenced by a sudden suppression of the trimer fluorescence. Gas pressures used in these experiments were 1000 Torr of rare gas and 1 Torr of halogen. Trimer and excimer fluorescence emanating from the cell was detected by a photomultiplier tube (PMT) after being dispersed by a 0.27-m monochromator whose output wavelength was set to coincide with the peak in the spectral band of the molecule of interest. The output was displayed on a fast oscilloscope and sampled by a boxcar averager. The response of the detection system was determined by use of a deuterium lamp (200 nm I X I350 nm) and a tungsten lamp (350 nm I X I 6 0 0 nm) that was traceable to an NBS standard, allowing the relative photon flux to be determined from the PMT output signal at any wavelength. Fluorescence spectra were recorded in order to determine the spectral bandwidths (fwhm) of each of the emitting species.
111. Optical Excitation XeCI*. An XeCl laser was used to photoassociatively excite the B state of XeCl at 308 nm.33-15 The same excimer pulse served to dissociate CI2 (creating two 0.77-eV chlorine atoms) and optically excite XeCI* during an Xe-CI collision. At the high xenon pressures, a large amount of Xe2C1(42r)was formed in a three-body process, and it radiated into an 86-nm-fwhm spectral band centered at 480 nm.l6 The effective lifetime of Xe2CI* is 60 ns, taking into account the radiative lifetime of 245 nsl' and a C12 quenching rate constant of 4 X cm3/s.17 This was observed experimentally. When the probe laser was fired 50 ns after the pump laser, immediate suppression of the trimer fluorescence was observed.'* Measurement of the suppression as a function of photon fluence has allowed the trimer absorption cross section to be determined at excimer wavelength^.^.'^ During
61. 2113. .. , . ..
(4) McCown, A. W.; Ediger, M. N.; Geohegan, D. B.; Eden, J. G. J . Chem. Phys. 1985, 82, 4862. (5) Eden, J. G.; Chang, R. S . F.; Palumbo, L. J. IEEE J . Quantum Electron. 1979, QE-15, 1146. (6) Wadt, W.R.; Hay, P. J. Appl. Phys. Lett. 19'17, 30, 573; J . Chem. Phys. 1978, 68, 3850. (7) Last, 1.: George, T. F. J . Chem. Phys. 1987, 87, 1183. (8) Huestis. D. L.: Schlotter. N. E. J . Chem. Phvs. 1978. 69. 3100. (9) Michels. H. H.; Hobbs, R. H.; Wright, L. A. Chem. Phys. Lett. 1977, 48, 158.
(IO) Bender, C. F.; Schaeffer, H. F., 111 Chem. Phys. Lett. 1978, 53,27. ( 1 1 ) Wadt, W. R. J . Chem. Phys. 1980, 73, 3915. (12) Preliminary results were first presented at the Conference on Lasers and Electro-Optics, Baltimore, MD, 1987; Paper WR-6.
0022-3654/90/2094-89I 3$02.50/0
(13) Inoue, G.; Ku, J. K.; Setser, D. W. J . Chem. Phys. 1982, 76, 733. (14) Inoue, G.; Ku, J. K.; Setser, D. W. J . Chem. Phys. 1984,80,6006. (15) McCown, A. W.; Eden, J. G. J. Chem. Phys. 1984,8/, 2933. (16) Marowsky, G.; Glass, G. P.; Smayling, . . M.; Tittel, F. K.; Wilson, W. L. J . Chem. Phys. 1981, 75, 1153. (17) Tang, K. Y.; Lorents, D. C.; Sharpless, R. L.; Huestis, D. L.; Helms,
D.; Durrett, M.; Walters, G. K. 33rd Gaseous Electronics Conference, Norman, OK, 1980; Paper FB-I. (18) For a more thorough description of the fluorescence suppression technique, see: McCown, A. W.; Ediger, M. N.; Stazak, S . M.; Eden, J. G. Phys. Rev. A 1983, 28, 1440. (19) McCown, A. W.; Godard, J. A. B. Conference on Lasers and Electro-optics, Baltimore, MD, 1987; Paper WR-6.
0 I990 American Chemical Society
8914
McCown
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 e
h = 388 nm
Kr2F 4+-+l2r
'
.
t
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1000KriI F2 h = 351 nm e
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.
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Time Delay, A t (ns)
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-
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Waveforms of (a) Kr2F*(42r I2r)and (b) KrF (C A ) emission. Firing an XeF laser 60 ns after an ArF laser resulted in suppression of the Kr2Ffluorescence and enhancement in the KrF. The peak intensitiesof the laser pulses were IXeF= 10 MW/cm2 and IAIF= 500 MW/cm2. Figure 1.
-
-
the probe laser pulse, not only did trimer fluorescence suppression take place but enhancement in the XeCl (B X) and (C A) intensities also resulted, indicating that the B and/or C state is a fragment of the trimer photodissociation. K r P . Although photoassociation at 248 nm can be used to excite the B state of KrF excitation with an ArF laser provides much larger excimer densities. Two 193-nm photons resonantly excited the Kr 6p[3/2I2 level, which radiated and was collisionally quenched to the krypton metastable.2' The KrF excimer was formed in a harpoon reaction between the metastable and a fluorine molecule. The trimer was then formed in a three-body process. An alternate route involves formation of the krypton dimer (Kr2*) through a three-body collision with the atomic metastable and F2 quenching of the dimer to form Kr2F(4V). At the krypton and fluorine pressures employed in these experiments (1000 and 1 Torr), quenching and three-body rates were very fast, and Kr2F* radiated into a 78-nm-fwhm band centered at 400 nm and had an effective lifetime of 75 A waveform of Kr2F* emission is presented in Figure la. The monochromator wavelength (388 nm) was chosen to allow some leakage of the ArF laser light (in second order) to be detected for timing purposes. The fluorescence peaked 50 ns after being initiated by the ArF laser and exhibited an exponential decay (T' 75 ns). The effect of firing an XeF laser (351 nm) 60 ns after the ArF laser is also seen in the upper set of waveforms. Immediate suppression of the Kr2F* fluorescence took place, corresponding to a depletion of the trimer population due to absorption. A slight recovery in the Kr2F*density was also observed. The lower set of waveforms (Figure 1 b) show the time history of the KrF(C) state (A = 280 nm). The intensity, and hence C state density, peaked -32 ns after the ArF laser was fired. The temporal dependence of the (B X) fluorescence was identical with the (C A) signal, although the peak intensity was much greater due to the shorter B state radiative lifetime and narrower emission band. At the firing of the XeF laser, an instantaneous increase in the (C A) and (B X) intensity occurred. In order to prove that this increase in the excimer population was due to Kr2F* photodissociation and not some other process, the height of the enhancement signal was measured as a function of time delay between the lasers, while holding the energy outputs of the
-
-
-
-
Enhancement in KrF (C A) fluorescence intensity sampled at 280 nm as a function of time delay between the ArF and XeF lasers. The data exhibit a peak at a time delay of 50 ns and an exponential decay constant of - 7 5 ns. Figure 2.
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(20) Schloss, J . H.; Geohegan, D. B.; Eden, J . G. Bull. Am. Phys. SOC. 1987, 32, 1230. (21) Geohegan, D. B.: McCown, A . W.;Eden, J . G. Phys. Rev. A 1986, 33, 269. (22) Luches, A.; Nassisi, V.;Perrone, A.; Perrone, M. R. Opt. Commun. 1982, 44, 109.
two lasers fixed. The time dependence of the enhanced signal identifies its precursor. Figure 2 gives the results of this experiment. The enhancement in the KrF(B) density peaked at a time delay of 5 0 ns and has a decay which matches that of the trimer fluorescence. Therefore, the observed increase in (B X) and (C A) fluorescence that occurs when Kr2F* absorption takes place at 35 1 nm is due to excitation of the trimer to a dissociative state which separates into the excimer molecule and a rare gas atom. Although not shown, photodissociating Xe2CI* at 351 nm gave identical results. This effect has also been recently observed in the photodissociation of Ar2F(421') at 248
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IV. Determination of the Branching Ratio Although it has been shown that Kr2F* and Xe2CI* absorptions at 351 nm result in the formation of KrF* and XeCI*, it is unclear what fraction of the excited trimer molecules dissociate to form the excimer. It was therefore desirable to determine the branching ratio for the process, especially as a function of wavelength, since absorption at certain wavelengths may excite upper trimer levels that do not dissociate. Calculating the branching ratio is not as simple as dividing the number density of created excimers by the number density of absorbing trimer molecules, since both the trimers and the excimers have finite lifetimes, and the excimer lifetime is comparable to the probe pulse width. Instead, the rate equations that describe the time rate of change of the trimer and excimer number densities must be solved and the calculated excimer density must be compared to the measured density to determine the branching ratio. Unfortunately, the excimer and trimer kinetics are not completely understood, and it is impossible to precisely model the systems. For many kinetics reactions, measurements of rate constants disagree by a factor of 2 or more,24 and many rate constants cannot be accurately determined at all. However, when a high-energy density probe pulse is used to completely dissociate all of the trimer molecules, the number of excimers produced is not a strong function of kinetics rate constants. In addition, by considering two possible kinetics scenarios and by observing the effect of varying rate constants on the calculated excimer density, upper and lower bounds on the branching ratios can be determined. The first case assumes that the excimer (B, C) levels are completely mixed25and act as one state E. The rate equations that describe the temporal evolution of the trimer and excimer levels during and after the firing of the probe laser are then given by (23) Hakuta, K.; Miki, S.;Takuma, H. J . Opt. SOC.Am. B 1!@8,5, 1261. (24) See, for example: Brau, C. A. In Excfmer Lasers: Rhodes, C. K., Ed.; Springer-Verlag: Berlin, 1984; pp 87-1 37. (25) Black, G.;Sharpless, R. L.; Lorents, D. C.; Huestis, D. L.; Gutcheck, R. A.; Bonifield, T. D.; Helms, D. A.; Walters, G.K. J . Chem. Phys. 1981, 75, 4840.
Production of Rare Gas-Halide Excimers
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 8915 100
B,C Mixed 0 80
3x10 -31
where Tis the number density of the trimer (Kr2F* or Xe2CI*), E is the number density of the mixed excimer, a is the photodissociation cross section at the probe laser wavelength, I is the probe laser beam intensity, h w is the photon energy, rT, is the effective lifetime of the trimer (75 ns for Kr2F* and 60 ns for Xe2CI*), k3 is the three-body trimer formation rate constant, M is the rare gas density, b is the branching ratio, and the excimer decay rate, ( T ~ , ) - ~is, given by -1+ - -kBC 1 1 _ - 1st kce 7C’ (3) TE‘ kBC I+-
-
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B mixing rate constants,
040
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EB - Ec (4) kBC EB and Ec are the energies of the lowest vibrational level of the B and C states, and (re.)-’ and (rCt)-l are the decay rates (including spontaneous emission and quenching) of the B and C states. This treatment assumes that the feeding term to the trimer has been depleted by the time the probe laser is fired and considers the excimer three-body loss rate separately from the other excimer loss rates. A considerable amount of effort has been given toward measuring the energy spacing in XeC1,26and a recent spectroscopic measurement2? has placed the C state lower in energy by 90 cm-l, giving a value of 0.65 for g. KrF has been relatively ignored in these types of studies, although a recent measurement by Yu and Setser28places the C state 120 f 50 cm-’ below the B state. The mixing rates are not known, although constants in the IO-” cm3/s range are generally quoted (usually for mixing species such as argon or neon, which are smaller than krypton29 or xenon14 and will give smaller mixing rates). By comparing the peak (temporal) (B X) intensity to the peak (C A ) intensity, a measurement of g was made for KrF. The intensities were spectrally integrated, and a correction was made for the contribution of (B A) intensity to the (C A) signal.30 With lifetimes of 6.8 ns for the B state3’ and 75 ns for the C ~ t a t e ,the ~ ~equilibrium .~~ ratio of C to B population was determined to be 1.2 (g = 0.8),corresponding to an energy separation of -40 cm-’ with the C state being lower. For an F2 quenching rate constant of 4 X cm3/s (ref 31) and a two-body Kr quenching rate constant of 1 X cm3/s (ref 33), the effective lifetime of the KrF B state is 5.2 ns and the effective lifetime of the C state is 17.2 ns. Therefore, for KrF, rE‘is equal to 8.4 ns. It must be noted that this calculation does not include the three-body loss rate (k3h@),which is included separately in ( I ) and (2). Strictly speaking, rE,is not the effective lifetime of
-
060
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kCB
kBc and kcB are the B C and C related by the Boltzmann factor
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(26) LeCalve, J.; Castex, M. C.; Jordan, B.; Zimmerer, G.; Moller, T.; Haaks, D. In Photophysics and Photochemistry above 6 eV; Lahmani, F., Ed.; Elsevier Science Publishers: Amsterdam, 1985; pp 639-649. Tellinghuisen, J.; McKeever, M. R. Chem. Phys. Len. 1980, 72,94. Bokor, J.; Rhodes, C. K. J . Chem. Phys. 1980, 73,2626. Yu, Y . C.; Setser, D. W.; Horiguchi, H. J . f h y s . Chem. 1983, 87, 2199. Brashears, H. C.; Setser, D. W. J . Phys. Chem. 1980, 84, 224. (27) Jouvet, C.; Lardeux-Dedonder, C.; Solgadi, D. Chem. Phys. Lett. 1989, 156. 569. (28) Yu, Y . C.;Setser, D. W. J . Phys. Chem. 1990, 94, 2934. (29) Takuma, H.; Ueda, K.; Hakuta, K.; Nishioka, H. Conference on Lasers and Electro-optics, Baltimore, MD 1987; Paper Tu02. (30) Julienne, P. S.;Krauss, M. Appl. fhys. Lett. 1979, 35, 5 5 . (31) Eden. J. G.;Waynant, R. W.; Searles, S. K.; Burnham, R . Appl. Phys. Lett. 1978, 32, 733. (32) Hay, P. J.; Dunning, T. H. J . Chem. Phys. 1977, 66, 1306. (33) Jacob, J. H.; Rokni, M.; Mangano, J. A.; Brochu, R. Appl. fhys. Lett. 1978, 32. 109.
0 20
0 00 0
5
10
15
M
25
30
Time (ns)
Figure 3. Calculated temporal history of the photodissociation-produced KrF relative to the initial Kr2F density for a 15-11sfwhm, I00 MW/cm* XeF probe laser pulse. The upper set of curves are solutions to eqs 1 and 2 while the lower set are solutions to eqs 5-7. The three-body trimer formation rate constant is varied as shown.
the excimer, but the effective lifetime in the absence of the three-body loss rate. For the case of XeCI, the radiative lifetimes are 1 I . 1 ns for the B state and 131 ns for the C statei4and the equilibrium ratio of C to B population is 1.55 (from ref 27). For a C12quenching rate constant of 4 X cm3/s (ref 14) and a xenon quenching rate cm3/s (first reference of ref 26), the effective constant of 4 X lifetime of the XeCl B state is 4.3 ns and the effective lifetime of the C state is 6.7 ns. Therefore, for XeCI, r g is equal to 5.5 ns. The second kinetics scenario treats B-C mixing explicitly. Since the C states are lower in energy than the B states, it is assumed that the C state feeds the trimer level34and the B state is produced by trimer photodissociation. The resulting rate equations are (5)
C dc = kBcM(B - gC) - - - k 3 W C dt Tc
(7)
where the mixing and three-body loss rates are again considered separately from the decay rates and a value of 5 X IO-” cm3/s was chosen for kBc. Equations 1 , 2, and 5-7 were solved by using a fourth-order Runge-Kutta algorithm. For the case of XeF photodissociation of Kr2F*, h w = 3.53 eV and a = 5.4 X cm2 (refs 1 and 19), while for Xe2CI a = I X IO-’’ cm2 (ref 4). The XeF pulse shape was nearly triangular with a 10-ns rise time and a 20-ns fall. Initially, the trimer density was set equal to 1.0 and the excimer density was 0. At t = 0, photodissociation of the trimer began, and the peak probe intensity was set equal to 100 MW/cm2, a (34) See: Daniiychev, V. A.; Dolgikh, V. A.; Kerimov, 0. M.; Samarin, A. Y . ;Tamanyan, G. Y . Sou. J . Quantum Electron. 1986, 16, 1395, where they measured a three-body loss rate from XeF(C) that is 5 times the loss rate from XeF(B).
McCown
8916 The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 100
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TABLE I: Emission Bands and Detection Constants A. nm A. SI R' 1.5 X I O B c 0.39 KrF(B) 248.4 1.3 X 0.83 KrF(C) 280.0 6.7 X lo6' 0.83 Kr2F 400.0 8.9 X IO7/ 0.96 XeCI(B) 307.9 7.6 X io6/ 0.99 XeCI(C) 345.0 4.1 X 106t 0.43 Xe,CI 480.0
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TABLE 11: Branching Ratio Data
1 00
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0
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Figure 4. Calculated temporal history of the photodissociation-produced XeCl relative to the initial Xe2CI density for a 15-ns-fwhm, 100 MW/
m
cm2 XeF probe laser pulse. The upper set of curves are solutions to eqs 1 and 2 while the lower set are solutions to eqs 5-7. The three-body trimer formation rate constant is varied as shown.
0.2
0
1 :I
200
value that experimentally resulted in complete suppression of both trimers. The branching ratio ( b ) was set equal to one, and calculations of the excimer and trimer densities as a function of time were made for different values of kj, since values for KrF in the (ref 35) to 9.7 X 10"' cm6/s (ref literature range from I x 31) whilevalues for XeCl range from 7.3 X (ref 17) to 1.4 X cm6/s (ref 36). A consequence of increasing the three-body trimer formation rate constant was that the calculated dependence of the trimer fluorescence suppression on laser fluence (+) did not follow a simple exponential decay as given in refs 1-3 and 18,where the amount of fluorescence suppression (FF) was given as FF = exp(-o+) (8) This effect was first reported by Schloss et al.,37who pointed out that the actual absorption cross sections may be a factor of 3 or 4 larger than those measured by using the fluorescence suppression technique, depending on the actual value of the three-body rate constant. In the calculations performed in this study, for each value of k3, the XeF photodissociation cross section was adjusted until the calculated suppression agreed with previous experimental results (given in refs 4 and 19). Model calculations of the photodissociation-produced KrF and XeCl densities are depicted in Figures 3 and 4,respectively. In both cases, the upper set of curves result from solving ( I ) and (2), the well-mixed case, while the lower set of curves are solutions to (5)-(7), and the total excimer density (B + C) is plotted. In Figure 3, k , has values of 0, 1 X IO-,!, 3 X and IO X cm6/s. It can be seen that the effect of increasing the three-body trimer formation rate constant is to shorten the time for the (B, C) state density to reach its peak, but there is little effect on the peak density. The peak KrF excimer density ranges from 0.58 to 0.63 for a trimer density of 1 .O (at the firing of the probe laser) (35) Morgan, W . L.; Szoke, A. Phys. Reu. A 1981, 23, 1256. (36) Moody, S. E.; Levin, L. A.; Center, R. E.; Ewing, J. J.; Klosterman, E. L. Report to DOE (Contract AC06-770P40037) Mathematical Sciences Northwest, 1982. (37) Schloss, J. H.; Jones, R. B.; Eden, J . G. International Conference on Lasers '89, New Orleans, LA, 1989; Paper HB-5.
!
1
240
.
1
1
1
280
1
320
1
360
Wavelength (nm)
Figure 5. A plot of branching ratio for formation of KrF(B,C) following Kr,F(421') absorption versus wavelength. The branching ratio is zero for wavelengths less than 250 nm and 0.9 f 0.1 for wavelengths above 300
nm.
and for a branching ratio of one. Also, there is not a significant difference between the results obtained from the two scenarios. From Figure 4,the peak XeCl excimer density ranges from 0.54 cm6/s and and 15 X 1 to 0.59 for k3 values of 0, 5 X an initial trimer density of 1.0. The branching ratio for the formation of excimers following trimer photodissociation was therefore determined by measuring the excimer density at its peak relative to the trimer population at the time the probe laser was fired and dividing that number by 0.6 for both KrF and XeCI. The relative peak excimer and trimer densities were determined from the PMT output signals after taking into account radiative lifetimes, spectral bandwidths, and system response. These data are given in Table I. V. Results and Discussion Experiments were performed using XeF, N2 (337 nm), XeCI, KrF, and ArF gas mixes in the probe laser. In the Kr2F*-KrF* studies, the ArF laser peak intensity was fixed at 500 MW/cm2 and the peak Kr2F* signal output was recorded. The probe laser was triggered at the point at which the trimer density peaked, and the trimer population was totally depleted. The resulting enhancements in both the B and C state signal outputs were recorded, and the branching ratio was calculated. This was repeated several times to ensure accuracy of results. A plot of branching ratio versus wavelength is shown in Figure 5, and the values are given in Table 11. For wavelengths above 300 nm, the branching ratio is nearly one and has no structure. For wavelengths less than 250 nm, the branching ratio is zero. There is some uncertainty in the value at 248 nm,since stimulated emission may have depleted the KrF(B) population so that no signal was seen. In the Xe2CI*-XeCI* case, the XeCl pump laser peak intensity was fixed at I25 MW/cm2, and the experiments were repeated. Branching ratios are plotted in Figure 6 and presented in Table
Production of Rare Gas-Halide Excimers 1 .o
l
0.8 -
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l
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 8917 I
l
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~
Xe2CI
0.6 0.4 -
-
-
-
0.2
0
1
1
'
1
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Wavelength (nm)
1
I
-
Figure 6. Branching ratios versus wavelength for Xe2C1(421') + hu XeCI(B,C). There is no data point at 308 nm because the probe laser
could not be operated at that wavelength.
(dashed lines) to take into account the results of these experiments. Since the XeCl C state lies below the B state, their positions have been reversed compared to Huestis's original curves, and the C is shown to feed the trimer. In an effort to experimentally reduce B-C mixing in order to determine whether the excimer B or C state was the photodissociation product, the xenon pressure was reduced to 300 Torr and the branching ratio experiment was repeated. The enhancement of the XeCI(B) state density was 4 times greater than the enhancement of the C-state density, indicating that the B state is probably the photofragment and is strongly repelled by the presence of a xenon atom. A definite answer to this question awaits comprehensive experiments at even lower pressures. The approximate position of the repulsive curve above the miminum of the 4 2 r potential well is realized by taking into account the fact that dissociation takes place for wavelengths larger than 300 nm. Therefore, the curve lies at most 4 eV above the minimum in the 4 2 r well. At wavelengths less than 250 nm, absorption takes place, but no excimer fluorescence is observed. This suggests the existence of a higher lying trimer state that is as yet unidentified or the possibility that the trimer is being photoionized, as was suggested in ref 4. Recent measurements of absorption in discharge pumped Ar2F, Kr2F, and Xe2C1(42r) as a function of wavelength have revealed at least one and perhaps a series of absorption bands to the blue of the (42r 921') band.39
-
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w
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4
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6
8
10
Separation, R (ao) Figure 7. Potential energy curves of Xe2CItaken from ref 38 which have been modified to take into account the results obtained from this study. Changes from the original are denoted by dashed lines. A curve that dissociates to the B state of the excimer has been added, as well as a higher lying excited state. 11. There is no data point at 308 nm because the probe laser could not be operated at 308 nm. The uncertainties given in both Figures 5 and 6 are a result of variations in the (B, C) enhancements. Comparing Figures 5 and 6 , one observes that the branching ratios as a function of wavelength are nearly identical for both Kr2F* and Xe2CI*. This suggests that the potential energy curves for Kr2F* and Xe2CI* are similar. The Xe2CI curves taken from Huestis et al.38are displayed in Figure 7, having been modified (38) Huestis, D.L.; Marowsky, G.; Tittel, F.K. In Excimer Lasers-1983; Rhodes, C. K., Egger, H., Pummer, H., Eds.; American Institute of Physics: New York, 1983;p 239.
VI. Conclusions In conclusion, absorption by the rare gas-halide trimers Kr2F and Xe2C1(421') for wavelengths longer than 300 nm results in the production of the B and/or C state of the associated excimer with a branching ratio of near unity. At wavelengths shorter than 250 nm, absorption takes place, but apparently to a bound trimer level. Future experiments using a dye laser probe will provide a continuous measurement of the branching ratio versus wavelength. It is hoped that these results will stimulate a renewed interest in the theory of trimer formation and kinetics and will as well generate interest in computing new potential curves. Acknowledgment. The excellent technical assistance of Jody Godard is gratefully acknowledged along with the glass-blowing expertise of Bill Fox and the typing skills of Lorraine Wilson and Becky Johnson. Several valuable discussions with Dennis Greene, Frank Feiock, Dave Hanson, Ed Salesky, Jeff Hay, Bill Wadt, Gary Eden, and Dave Geohegan are also appreciated. This work was supported by the Department of Energy under the Inertial Confinement Fusion program. (39)McCown, A. W.; Greene, D. P. In Proceedings of the International Conference on Lasers '89; Harris, D. G.,Shay, T. M., Eds.; STS Press: McLean, VA, 1990 pp 161-113. (40) Huestis, D. L.;Marowsky, G.;Tittel, F. K. In Excimer Lasers; Rhodes, C. K., Ed.; Springer-Verlag: Berlin, 1984;p 193.
