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Gerade Rydberg states of Xe2 probed by laser spectroscopy in the afterglow of a Xe microplasma: Ωg←a3Σu+(1u,Ou-) transitions in the near-infrared (675–800 nm)
Chemical Physics Letters 422 (2006) 372–377 www.elsevier.com/locate/cplett
Gerade Rydberg states of Xe2 probed by laser spectroscopy a3Rþ in the afterglow of a Xe microplasma: Xg u ð1u ; Ou Þ transitions in the near-infrared (675–800 nm) C.J. Wagner *, J.G. Eden Laboratory for Optical Physics and Engineering, Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801, USA Received 15 August 2005; in final form 17 January 2006 Available online 23 March 2006
Abstract Gerade Rydberg states of Xe2 lying 12 500–14 800 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable levels of the dimer have been observed by laser spectroscopy in the afterglow of a Xe microplasma with a volume of 160 nL. Photoexcitation of Xg a3 Rþ u ð1u ; Ou Þ (Rydþ berg Rydberg) state transitions at small internuclear separations ðR ’ Re ðXe2 ÞÞ while monitoring the suppression of þ Xe2 ðOþ u ! Og Þ spontaneous emission in the ultraviolet confirms the presence of an extensive series of red-degraded vibrational bands in the 675–800 nm region that are tentatively identified with an X = 1 upper state with a molecular ion core having predominantly ˚ Te = 12 980 ± 70 cm1, B2P3/2g character. Spectral simulations yield the following constants for the 1g state: R0e ¼ 3:49 0:1 A, 1 1 1 0 0 0 0 xe ¼ 122 3 cm , xe xe ¼ 0:9 0:2 cm , and De ¼ 2250 70 cm . 2006 Elsevier B.V. All rights reserved.
The electronic structure of Xe2 continues to be of both fundamental and applied interest. From a spectroscopic perspective, the spin-orbit interaction in Xe2 is the largest of the rare gas dimers (1 eV) and its state designations are best described by Hund’s case (c) notation. As the heaviest member of a homologous family of diatomics whose electronic excited states are entirely Rydberg in character, Xe2 is important to the study of Z-dependent variations in the coupling of the Rydberg electron to its ion core. Insofar as its technological value is concerned, Xe2 is at the heart of the plasma display panel as well as a variety of lamps owing to the efficiency of the lowest Oþ u and 1u states as radiators in the vacuum ultraviolet (VUV: 172 nm). Despite more than three decades of spectroscopic experiments, however, the structure of the rare gas dimer Xe2 at small internuclear separations ðR Re ðXeþ 2 ÞÞ remains poorly characterized [1]. Since the offset between the equi*
Corresponding author. Fax: +1 217 244 5422. E-mail address: [email protected] (C.J. Wagner).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.01.111
librium internuclear separation, Re, for the ground state of þ the dimer ion and that for the neutral species ðX1 Rþ g ðOg ÞÞ ˚ , spectroscopic experiments in which the jet-cooled is J 1 A dimer is photoexcited from the van der Waals minimum are constrained to examining the high v 0 portions of the Rydberg interatomic potentials. As discussed in detail by Jonin and Spiegelmann [2], both gerade and ungerade Rydberg states have been accessed by one or two photon transitions and among the states examined thus far are those correlated, in the separated atom limit, with Xe(1S0) and either the 6s,6s 0 ,6p,5d, or 4f states of Xe. Virtually all of these experiments probed the dimer in the Franck–Condon region centered on Re ðXOþ g Þ. In contrast, few experimental studies of Xe2 Rydberg structure have been reported for ˚ ) where configuration interaction is minsmall R (< 3.5–4 A imal and the excited dimer most closely resembles its ion core. In 1986, Killeen and Eden [3] reported the observation of Rydberg series for all of the rare gas dimers (except Rn2). For Xe2, bands associated with six Rydberg– Rydberg transitions were evident, the strongest of which peaked at 549.4 nm [4]. At the time, the existence of a
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
the afterglow, a fraction of the Xe2 a3 Rþ u ð1u ; Ou Þ metastable molecules formed in the active plasma was photoexcited by the arrival of a 7 ns FWHM pulse from a Nd:YAGpumped dye laser. Vacuum ultraviolet fluorescence from the Xe2 ð1ÞOþ u state was monitored by a gated solar blind photomultiplier that was energized 1.37 ls prior to the arrival of the dye laser pulse. Accurately measuring the suppression of the VUV emission by an absorptive transition of Xe2 ða3 Rþ u Þ requires that the dye laser beam cross-section be shaped so as to match the microplasma dimensions. This was accomplished with two cylindrical lenses (focal lengths of –120 cm and 40 cm, respectively) and the alignment procedure benefited significantly from imaging the discharge gap onto a screen. The dye laser wavelength was calibrated against uranium lines and the linewidth of the laser was nominally 1 cm1. Further details concerning the characteristics and performance of the experiment will be presented elsewhere. þ Fig. 1 illustrates Xe2 ðOþ u ! Og Þ emission waveforms that are representative of those observed throughout the experiments. Both waveforms are the average of 100 measurements and the dye laser was tuned to 754.0 nm, the wavelength of one of the prominent bandheads in the spectrum to be discussed later. Notice that the magnitude of the fluorescence suppression in the figure is 37% and that, following the dye laser pulse, the Xe2 fluorescence eventually recovers. The latter is the result of the uninterrupted formation of Xe2 ða3 Rþ u Þ molecules from Xe(6s) atoms produced in the active plasma. It is interesting to note that the suppression waveform of Fig. 1 actually overshoots the unperturbed (‘No laser pulse’) intensity trace and maintains a slightly higher intensity for several hundred ns. This effect is real and its origin lies in the fate of Xe2 molecules removed from the a3 Rþ u ð1u ; Ou Þ states by photoabsorption. Predissociation of the Xe2 Rydberg state(s) populated by the dye laser pulse, followed by collisional relaxation of
0.45 0.4
400 Torr Xe
0.35 Relative Amplitude
quasicontinuum in the near-infrared with ‘. . .distinct features. . .hav[ing] a periodicity of 115±14 cm1. . .’ [3] was noted and assumed to be associated with a vibrational sequence but this structure was neither discussed further nor analyzed. Aside from the work of Ref. [3], other experiments to have probed the Franck–Condon region R Re ðXeþ 2 Þ are those of Jonin et al. [5–7] and Arai et al. [8] Adopting a fluorescence suppression technique similar to that applied to Kr2 (Ref. [9]) and developed earlier for the rare gas dimer ions [10], Ne2 (Ref. [11]) and the rare gas-halide diatomics and triatomics [12,13], the French group also observed the molecular features near 549.4 nm reported previously in Refs. [3], [4] but detected additional structure superimposed onto the band profile. More significantly, absorptive transitions originating from Xe2 ðOþ uÞ were reported in Ref. [6]. Measurements by Arai and coworkers [8] of Xe2 excited state absorption in the nearinfrared revealed vibrational structure in the 1.0–1.1 lm region. This Letter describes experiments in which gerade Rydberg states of Xe2 lying 12 500–14 800 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable states have been detected by laser spectroscopy in the afterglow of a Xe microplasma [14] with a volume of 160 nL and operating at 400 Torr. Photoexcitation of Xe2 ð1u ; O u Þ while monitoring the suppresþ sion of Oþ ! XO spontaneous emission in the vacuum u g ultraviolet (VUV) and deep-UV confirms the existence of molecular structure near 700 nm as reported originally in Ref. [3]. Specifically, a well-developed series of reddegraded vibrational bands is observed extending from 675 nm to beyond 800 nm. These bands, constituting the most extensive vibrational structure observed to date for any of the rare gas dimers at small R, appear to be associated predominantly with a single upper electronic state. The proposed identity of this level, as well as the derived vibrational constants for the Rydberg Rydberg transitions, are discussed. The experimental approach implemented for these studies is based on the production of excited Xe2 molecules in the a3 Rþ u ð1u ; Ou Þ metastable states in a microplasma. After exploring several microdischarge structures and geometries, a simple, yet reliable and robust, design was found to consist of 1.5 mm diameter tungsten rods separated by 90 lm. Reproducible glow discharges were obtained in Xe at pressures of several hundred Torr by roughening the end faces of each rod and slightly rounding the edges in order to reduce field enhancement. These measures resulted in improved stability and ignition characteristics for the discharge which operated at a voltage and current density of 220 V and 226 mA cm2, respectively, for a Xe pressure of 400 Torr. Despite the 16:1 aspect ratio of the microplasma geometry, the discharge is a spatially uniform glow and is confined to the 160 nL volume between the electrodes [14]. For the data reported here (pXe = 400 Torr), the microplasma was crowbarred for 0.5 ls at a pulse repetition frequency of 10 Hz and, approximately 90 ns into
373
No laser pulse With laser
0.3
λdye=754.0 nm
0.25 0.2 0.15 0.1
Signal
0.05 0 -0.05
0
500
1000 1500 Time (ns)
2000
2500
Fig. 1. Waveforms, representative of those observed in these experiments, illustrating the suppression of Xe2 VUV emission by a dye laser pulse at kdye = 754.0 nm. The Xe pressure is 400 Torr and each waveform represents the average of 100 measurements.
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
the excited atomic fragment, will result in a fraction of the photoexcited molecules returning eventually to the a3 Rþ u state. Consider, for example, the ‘Signal’ waveform of Fig. 1 which represents the difference between the Xe2 fluorescence intensity traces recorded with, and without, the presence of the dye laser pulse. Roughly speaking, the early portion of the waveform corresponds to that time period in which population is removed from the Xe2 ða3 Rþ u Þ state, whereas a portion of the recovery of the VUV fluorescence and the negative signal evident beyond 550 ns reflect the return of a portion of that population. Obtaining reliable, unsaturated fluorescence suppression spectra requires that the variation of the degree of suppression with laser fluence be taken into account. Representative data, the results of 3150 measurements for the laser wavelength k fixed at 784.0 nm, are shown in Fig. 2. For the spectra reported here, the laser energy fluence delivered to the microplasma was fixed at 2 mJ cm2 (2.7 lJ per pulse for a plasma cross-section of 1.35 · 103 cm2) which is represented on the abscissa of Fig. 2 by a relative pulse energy of 0.4 and corresponds to a fluorescence suppression of 16 ±2%. The fluorescence suppression spectrum of Xe2 a3 Rþ u ð1u ; O u Þ, recorded in the 675–830 nm wavelength interval for a Xe pressure of 400 Torr, is indicated by the blue curve in Fig. 3. Each of the points comprising the spectrum is the average of 150 measurements at that wavelength. The overall spectrum has been corrected for the variation of the dye laser pulse energy with wavelength but has not as yet been normalized to the absolute excimer absorption cross-sections. The result (not illustrated in Fig. 3) is to flatten somewhat the spectral profile, particularly in the 12 500–13 000 cm1 region. A well-developed series of red-degraded vibrational bands is observed with individual bands exhibiting modulation depths as large as 60%. For comparison, a digitized segment of the absorption spectrum in 2310 Torr of Xe reported by Killeen and Eden in 1986 (Ref. [3]) is also given in Fig. 3 (red trace). It is immediately evident that all of the significant features in
0.4
Suppression
0.3 0.2 0.1 λ=784nm
0 − 0.1 0
0.5
1 2 1.5 Relative Pulse Energy
2.5
Fig. 2. Dependence of Xe2 fluorescence suppression on the relative dye laser pulse energy for a laser wavelength of 784 nm. Results are given for 3150 measurements at energy fluences, incident on the microplasma, up to 13 mJ cm2 per pulse.
3
14.5
-1
Wavenumbers (10 cm ) 14 13.5 13 12.5
12
0.22 0.2
Relative Suppression
374
0.18
0.3
Xe 400 Torr
Ref. 3
0.25
0.16
0.2
0.14 0.12
0.15
0.1 0.1
0.08 0.06
0.05 680
700
720 740 760 780 Wavelength (nm)
800
820
Fig. 3. Comparison of the laser-induced fluorescence suppression spec trum of Xe2 a3 Rþ u ð1u ; Ou Þ in the 675–830 nm region (blue curve) with the absorption spectrum reported in 1986 by Ref. [3] (in red). Each point in the suppression spectrum is the average of 150 measurements, and the overall spectrum is the composite of three segments, each of which required a separate laser dye.
the spectrum of Ref. [3] are matched by corresponding bands in the present data. One concludes that the ‘. . .distinct features at the peak of the near-infrared, quasicontinuum. . .’ observed in Ref. [3] are, indeed, real. Fewer than 10 vibrational bands lying between 730 and 770 nm were discernible in the electron beam experiments [3] whereas at least 20 distinct bands are detected in the present study. Relative to Ref. [3], the improved development of the vibrational structure reported here is attributable to several factors, including a significantly increased S/N ratio, a reduction in the Xe pressure by a factor of almost six, and a higher gas temperature. Analysis of the entire microplasma assembly, comprising the tungsten electrodes, ceramic (Macor) support structure, and the gas itself, with a one-dimensional thermal model predicts the average gas temperature in the microplasma to be 923 K for a steady-state current density of 0.23 A cm2 and a power input to the plasma of 880 mW (specific power loading of 5.5 kW cm3 to a plasma volume of 1.6 · 104 cm3). This situation contrasts with the low duty cycle, electron beam-driven absorption experiments of Ref. [3] in which the gas temperature did not rise far above ambient. Much of the structure in the experimental spectrum (Fig. 3) has been reproduced by simulations in which the upper and lower electronic states are represented by Morse potentials. Although the Morse function is known to be a poor representation of rare gas dimer Rydberg states at larger internuclear separations, itiis acceptable for a limited h 2 þ range in R near Re Xeþ X R . The observed transitions 1u 2 2
are assumed to terminate at a gerade state of Xe2 lying 12 500–13 500 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable levels, and the vibrational constants for the lower state were taken to be those proposed by Jonin and Spiegelmann [2]
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
for the 1(1u) state: x00e ¼ 109:0 cm1 and x00e x00e ¼ 1:14 cm 1 . More than 104 simulations were performed in which x0e ; x0e x0e ; and DR R0e R00e were iterated over the intervals ˚ , respectively. 80–140 cm1, 0.7–1.5 cm1, and 0.05–0.4 A For each set of spectroscopic constants, wavefunctions were calculated for both states and the Franck–Condon factors (FCFs) determined for v 0 , v00 6 20. Calculations ignored rotational effects but assumed the a3 Rþ u ð1u Þ vibrational population to be equilibrated and, thus, represented by a Boltzmann distribution for a temperature between 300 and 900 K. Of the 104 trials, 19 sets of constants provided an acceptable match to the experimental spectrum and the best fit yields the following constants for the upper state: x0e ¼122 3 cm1, x0e x0e ¼ 0:9 0:2 cm1, T 0e ¼ 12 980 ˚ , where the indicated 70 cm1, and DRe = 0.3 ± 0.1 A uncertainties represent one standard deviation determined from the 19 parameter sets. A comparison of the experimental and calculated spectra is presented in Fig. 4 for a gas temperature of 900 K. It is evident that the bandhead positions for the simulated spectrum, as well as the overall intensity profile at low photon energies (12 500– 13 000 cm1), match those of the experimental spectrum. The calculations have been less successful in reproducing weaker features and relative band strengths in the 13 100– 13 600 cm1 interval, but the abrupt loss of intensity for the vibrational band peaking at 14 010 cm1 is likely the result of a perturbation (cf. Fig. 4 and Ref. [2]). Also, a continuum appears to underlie the bound–bound structure in the short wavelength portion (13 700–14 800 cm1) of the experimental spectrum but has not thus far been incorporated into the simulations. The position and structure of the gerade Rydberg state of Xe2 determined from the simulations are also consistent with the predictions of pseudopotential hole-particle calculations of the dimer state structure reported in Ref. [2]. As noted earlier, Jonin and Spiegelmann have calculated the
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interatomic potentials for both gerade and ungerade Rydberg states correlated, in the separated atom limit, with Xe (5p6 1S0)+ Xe (6s, 6p, 5d, 7s, 7p). Of particular relevance to the present spectroscopic measurements are the strongly bound Xe2 electronic states derived from Xe (1S0)+ Xe (6p). Gerade Rydberg states determined in Ref. [2] to lie 10 000–16 000 cm1 above T e ða3 Rþ u Þ of the dimer are illustrated in Fig. 5. Only two states, ð3ÞOþ g and 5(1g), are predicted to exist within ± 800 cm1 (± 0.1 eV) of the value of the upper state Te reported here (12 980 cm1). As discussed in Ref. [2], the constants for both states are expected to be similar, and Table 1 compares these theoretical values with those inferred from the experimental spectra. As will be discussed in more detail later, the dissociation energy D0e given in Table 1 is calculated by assuming Xe* (6p[5/2]2)+ Xe 5p6 1S0 to be the separated atom limit for the upper state associated with the observed transitions. Since our experiments directly yield only DRe, the value of R0e in ˚ , as proTable 1 is based upon taking R00e ð1u Þ to be 3.19 A posed in Ref. [2]. Given this assumption, the inferred value ˚ ) is in remarkable agreement with those of R0e (3.49 ± 0.1 A calculated by Jonin and Spiegelmann [2] for the candidate upper states ðð3ÞOþ g andð5Þ1g Þ mentioned earlier. In Ref. ˚ ) and 6.69a0 [2], R0e is determined to be 6.61a0 (3.50 A ˚ ) for the Oþ and 1g states, respectively, which reflect (3.54 A g the predominant contribution (70–75%) of the Xeþ 2 ðB2 P3=2g Þ state to the structure of the molecular ion core [15]. Dimer Rydberg states built on the ground state of the molecular ion X 2 Rþ are generally characterized by 1u 2 Re 6a0 (Ref. [2]). Of the other spectroscopic constants for the 3ðOþ g Þ and 5(1g) states calculated in Ref. [2], the anharmonic constant x0e x0e for 3ðOþ g Þ agrees with the experimental value to within the estimated uncertainty, whereas the theoretical values of Te, xe, and De for both states differ from the 20 5d
18
Energy (103 cm–1 )
6p
Relative Intensity
Experiment
Present Experiment
16
6p[5/2]2 6s
14 Ref. 16 (3)0g+ (5)1g
12 10
Simulation T=900 K
8
2
3
4
5
6
7
8
9
10
Internuclear Separation R(Å) 12000
12500
13000
13500
14000
14500
15000
Wavenumbers (cm−1) Fig. 4. (Top) Portion of the experimental suppression spectrum, smoothed relative to that of Fig. 2; (Bottom) Simulation of the spectrum for a gas temperature of 900 K. Most of the features in the experimental spectrum are tentatively assigned to 5ð1g Þ a3 Rþ u ð1u ; Ou Þ transitions of Xe2.
Fig. 5. Partial energy level diagram of Xe2 (adapted from Ref. [2]), indicating the location of the Rydberg state observed in the present experiments (red curve) as compared to the 5(1g) position calculated by Jonin and Spiegelmann (Ref. [2]). The extrapolated potential proposed by Dimov et al. (Ref. [16]) is also indicated (blue curve), and the ordinate (energy) is referenced to the value of Te for the 1ðO u Þ state of Xe2.
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C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
Table 1 Spectroscopic constants for selected gerade states of Xe2, compared with values inferred from experimental spectra Spectroscopic constant ˚) Re (A Te (cm1)c xe (cm1) xexe (cm1) De (cm1)
Electronic state (3) Oþ g (5) 1g
Ref. [2] 3.50 3.54
(3) Oþ g (5) 1g
12395.0 12 388.0
(3) Oþ g (5) 1g
94.0 88.0
(3) Oþ g (5) 1g (3) Oþ g (5) 1g
1.18 1.26
Ref. [16]a
Present work 3.49 ± 0.1b
3.21 ± 0.03 12980 ± 70 11 756d 122 ± 3 128.6 ± 2.2 0.9 ± 0.2 1.587 ± 0.069 2250 ± 70d
2700
3470.4 ± 23.4
a Assumes that the constants reported in Ref. [16] are those for the (5)1g Rydberg state of Xe2. b ˚ (cf. Ref. [2]). Assumes R00e ¼3:19A c Relative to T e 1ðO u Þ — see Ref. [2]. d This value assumes Xe 6p[5/2]2 + Xe 5p6 1S0 to be the separated atom limit for the observed state.
experimental results by 5%, 30–40%, and [20%, respectively. It is clear, then, that the spectroscopic constants determined for the state responsible for the observed spectrum (Fig. 3) are quite close to those reported by Jonin and Spiegelmann for the 3ðOþ g Þ and 5(1g) states. However, differentiating between the Oþ g and 1g states as the upper level for the observed transitions is not possible at present and a definitive assignment will require the acquisition of rotationally-resolved spectra. In view of the strength of the observed transitions1 and the fact that ð3ÞOþ 1ðO g uÞ transitions are strictly forbidden, the assignment of 5ð1g Þ a3 R þ u ð1u ; Ou Þ is slightly favored. Nevertheless, this must be viewed as being tentative pending the availability of higher resolution spectra. We note that the choice of Xe 6p[5/2]2 + Xe 5p6 1S0 as the separated atom limit for the experimentally-observed state predicts the existence of a barrier in the potential, and a higher-lying asymptote would yield a value for De closer to the theoretical value. It should also be mentioned that, in 1994, Dimov et al. [16] observed a strongly bound dimerexcited state derived from Xe(5p6 1S0) and either Xe 6p 12 1 or 6p 52 2 . Resonantly-enhanced, multiphoton ionization (REMPI) experiments involving two photon excitation of the cooled dimer detected spectral features extending over a 350 cm1 wide region centered at 77 500 cm1, and tentatively assigned 0 to Oþ XOþ g g transitions terminating on 25 6 v 6 38 vibrational levels. Franck–Condon calculations suggested the upper state to be built on the ground state molecular ion X2 Rþ core, and the Dunham coefficients were deter1u 2 mined to be T 0e + Y0,0 = 74 846.15 ± 23.2 cm1, Y1,0
ð x0e Þ ¼ 128:6 2:2 cm1, Y2,0 ð x0e x0e Þ ¼ 1:587 0.069 cm1, and D0e ¼ 3470:4 23.4 cm1. Since only the high v 0 (P25) portion of the interaction potential was observed, extrapolation to the region near the potential minimum ðR R0e Þ was necessary, and the result reported in Ref. [16] is compared in Fig. 5 with the potential derived from the spectra reported here. Both interatomic potentials are portrayed by the Morse function and the solid and dashed portions of the red curve (present work) represent the experimentally-observed and extrapolated regions, respectively. Although the state reported by Dimov et al. [16] lies close in energy to the Rydberg level observed in the present experiments and a portion of the discrepancies between the two sets of constants in Table 1 are attribut able to the uncertainty in T e a3 Rþ u ð1u ; Ou Þ and the extrapolation over v 0 required in Ref. [16] to estimate spectroscopic parameters, the substantial differences in R0e and D0e suggest that the REMPI experiments observed a state distinct from that reported here. In summary, red-degraded vibrational bands associated with Rydberg Rydberg state absorptive transitions of Xe2 have been observed in the 675–800 nm region by laser-induced fluorescence suppression spectroscopy. Tentatively assigned to 5ð1g Þ a3 R þ u ð1u Þ transitions of the dimer, the observed spectrum exhibits the most extensive vibrational structure for anyiof the rare gas observed h 2 þ dimers at small R R ffi Re Xeþ ðX R1u Þ . This is a result 2 2 of the gas temperature in the plasma (900 K) and the ˚ ) between the equilibrium internuclear large offset (0.3 A separations (Re) for the two electronic states responsible for the spectrum. Simulations of the experimental spectra yield spectroscopic constants that are consistent with pseudopotential hole-particle theory. Acknowledgements The technical assistance of K. Collier and N. Ostrom, as well as the support of the US Air Force Office of Scientific Research under Grant No. F49620-03-1-0391, are gratefully acknowledged. References [1] [2] [3] [4]
[5] [6] [7] [8]
1 By measuring the dependence of the fluorescence suppression on laser intensity at several laser probe wavelengths, the absorption cross-sections for the Rydberg a3 Rþ u ð1u ; Ou Þ transitions have been measured to be 1017 cm2. These experiments will be described in detail elsewhere.
[9] [10]
M.L. Ginter, J.G. Eden, Can. J. Chem. 82 (2004) 762. C. Jonin, F. Spiegelmann, J. Chem. Phys. 117 (2002) 3059. K.P. Killeen, J.G. Eden, J. Chem. Phys. 84 (1986) 6048. The first indications of this band were observed by E. Zamir, D.L. Huestis, H.H. Nakano, R.M. Hill, D.C. Lorents, IEEE J. Quantum Electron. 15 (1979) 281. C. Jonin, P. Laporte, R. Saoudi, J. Chem. Phys. 108 (1998) 480. C. Jonin, P. Laporte, F. Spiegelmann, Chem. Phys. Lett. 308 (1999) 13. C. Jonin, E. Descroix, P. Laporte, F. Spiegelmann, Chem. Phys. Lett. 300 (2000) 595. S. Arai, T. Oka, M. Kogoma, M. Imamura, J. Chem. Phys. 68 (1978) 4595. M. Hemici, R. Saoudi, E. Descroix, E. Audouard, P. Laporte, F. Spiegelmann, Phys. Rev. A 51 (1995) 3351. A.W. McCown, M.N. Ediger, S.M. Stazak, J.G. Eden, Phys. Rev. A 28 (1983) 1440.
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377 [11] S.B. Kim, D.J. Kane, J.G. Eden, Phys. Rev. Lett. 68 (1992) 1311. [12] A.W. McCown, M.N. Ediger, J.G. Eden, Phys. Rev. A 28 (1983) 3365. [13] A.W. McCown, M.N. Ediger, D.B. Geohegan, J.G. Eden, J. Chem. Phys. 82 (1985) 4862; D.B. Geohegan, J.G. Eden, Chem. Phys. Lett. 139 (1987) 519.
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[14] C.J. Wagner, J.G. Eden, IEEE Trans. Plasma Sci. 33 (2005) 568. [15] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [16] S.S. Dimov, J.Y. Cai, R.H. Lipson, J. Chem. Phys. 101 (1994) 10313.
Gerade Rydberg states of Xe2 probed by laser spectroscopy a3Rþ in the afterglow of a Xe microplasma: Xg u ð1u ; Ou Þ transitions in the near-infrared (675–800 nm) C.J. Wagner *, J.G. Eden Laboratory for Optical Physics and Engineering, Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801, USA Received 15 August 2005; in final form 17 January 2006 Available online 23 March 2006
Abstract Gerade Rydberg states of Xe2 lying 12 500–14 800 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable levels of the dimer have been observed by laser spectroscopy in the afterglow of a Xe microplasma with a volume of 160 nL. Photoexcitation of Xg a3 Rþ u ð1u ; Ou Þ (Rydþ berg Rydberg) state transitions at small internuclear separations ðR ’ Re ðXe2 ÞÞ while monitoring the suppression of þ Xe2 ðOþ u ! Og Þ spontaneous emission in the ultraviolet confirms the presence of an extensive series of red-degraded vibrational bands in the 675–800 nm region that are tentatively identified with an X = 1 upper state with a molecular ion core having predominantly ˚ Te = 12 980 ± 70 cm1, B2P3/2g character. Spectral simulations yield the following constants for the 1g state: R0e ¼ 3:49 0:1 A, 1 1 1 0 0 0 0 xe ¼ 122 3 cm , xe xe ¼ 0:9 0:2 cm , and De ¼ 2250 70 cm . 2006 Elsevier B.V. All rights reserved.
The electronic structure of Xe2 continues to be of both fundamental and applied interest. From a spectroscopic perspective, the spin-orbit interaction in Xe2 is the largest of the rare gas dimers (1 eV) and its state designations are best described by Hund’s case (c) notation. As the heaviest member of a homologous family of diatomics whose electronic excited states are entirely Rydberg in character, Xe2 is important to the study of Z-dependent variations in the coupling of the Rydberg electron to its ion core. Insofar as its technological value is concerned, Xe2 is at the heart of the plasma display panel as well as a variety of lamps owing to the efficiency of the lowest Oþ u and 1u states as radiators in the vacuum ultraviolet (VUV: 172 nm). Despite more than three decades of spectroscopic experiments, however, the structure of the rare gas dimer Xe2 at small internuclear separations ðR Re ðXeþ 2 ÞÞ remains poorly characterized [1]. Since the offset between the equi*
Corresponding author. Fax: +1 217 244 5422. E-mail address: [email protected] (C.J. Wagner).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.01.111
librium internuclear separation, Re, for the ground state of þ the dimer ion and that for the neutral species ðX1 Rþ g ðOg ÞÞ ˚ , spectroscopic experiments in which the jet-cooled is J 1 A dimer is photoexcited from the van der Waals minimum are constrained to examining the high v 0 portions of the Rydberg interatomic potentials. As discussed in detail by Jonin and Spiegelmann [2], both gerade and ungerade Rydberg states have been accessed by one or two photon transitions and among the states examined thus far are those correlated, in the separated atom limit, with Xe(1S0) and either the 6s,6s 0 ,6p,5d, or 4f states of Xe. Virtually all of these experiments probed the dimer in the Franck–Condon region centered on Re ðXOþ g Þ. In contrast, few experimental studies of Xe2 Rydberg structure have been reported for ˚ ) where configuration interaction is minsmall R (< 3.5–4 A imal and the excited dimer most closely resembles its ion core. In 1986, Killeen and Eden [3] reported the observation of Rydberg series for all of the rare gas dimers (except Rn2). For Xe2, bands associated with six Rydberg– Rydberg transitions were evident, the strongest of which peaked at 549.4 nm [4]. At the time, the existence of a
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
the afterglow, a fraction of the Xe2 a3 Rþ u ð1u ; Ou Þ metastable molecules formed in the active plasma was photoexcited by the arrival of a 7 ns FWHM pulse from a Nd:YAGpumped dye laser. Vacuum ultraviolet fluorescence from the Xe2 ð1ÞOþ u state was monitored by a gated solar blind photomultiplier that was energized 1.37 ls prior to the arrival of the dye laser pulse. Accurately measuring the suppression of the VUV emission by an absorptive transition of Xe2 ða3 Rþ u Þ requires that the dye laser beam cross-section be shaped so as to match the microplasma dimensions. This was accomplished with two cylindrical lenses (focal lengths of –120 cm and 40 cm, respectively) and the alignment procedure benefited significantly from imaging the discharge gap onto a screen. The dye laser wavelength was calibrated against uranium lines and the linewidth of the laser was nominally 1 cm1. Further details concerning the characteristics and performance of the experiment will be presented elsewhere. þ Fig. 1 illustrates Xe2 ðOþ u ! Og Þ emission waveforms that are representative of those observed throughout the experiments. Both waveforms are the average of 100 measurements and the dye laser was tuned to 754.0 nm, the wavelength of one of the prominent bandheads in the spectrum to be discussed later. Notice that the magnitude of the fluorescence suppression in the figure is 37% and that, following the dye laser pulse, the Xe2 fluorescence eventually recovers. The latter is the result of the uninterrupted formation of Xe2 ða3 Rþ u Þ molecules from Xe(6s) atoms produced in the active plasma. It is interesting to note that the suppression waveform of Fig. 1 actually overshoots the unperturbed (‘No laser pulse’) intensity trace and maintains a slightly higher intensity for several hundred ns. This effect is real and its origin lies in the fate of Xe2 molecules removed from the a3 Rþ u ð1u ; Ou Þ states by photoabsorption. Predissociation of the Xe2 Rydberg state(s) populated by the dye laser pulse, followed by collisional relaxation of
0.45 0.4
400 Torr Xe
0.35 Relative Amplitude
quasicontinuum in the near-infrared with ‘. . .distinct features. . .hav[ing] a periodicity of 115±14 cm1. . .’ [3] was noted and assumed to be associated with a vibrational sequence but this structure was neither discussed further nor analyzed. Aside from the work of Ref. [3], other experiments to have probed the Franck–Condon region R Re ðXeþ 2 Þ are those of Jonin et al. [5–7] and Arai et al. [8] Adopting a fluorescence suppression technique similar to that applied to Kr2 (Ref. [9]) and developed earlier for the rare gas dimer ions [10], Ne2 (Ref. [11]) and the rare gas-halide diatomics and triatomics [12,13], the French group also observed the molecular features near 549.4 nm reported previously in Refs. [3], [4] but detected additional structure superimposed onto the band profile. More significantly, absorptive transitions originating from Xe2 ðOþ uÞ were reported in Ref. [6]. Measurements by Arai and coworkers [8] of Xe2 excited state absorption in the nearinfrared revealed vibrational structure in the 1.0–1.1 lm region. This Letter describes experiments in which gerade Rydberg states of Xe2 lying 12 500–14 800 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable states have been detected by laser spectroscopy in the afterglow of a Xe microplasma [14] with a volume of 160 nL and operating at 400 Torr. Photoexcitation of Xe2 ð1u ; O u Þ while monitoring the suppresþ sion of Oþ ! XO spontaneous emission in the vacuum u g ultraviolet (VUV) and deep-UV confirms the existence of molecular structure near 700 nm as reported originally in Ref. [3]. Specifically, a well-developed series of reddegraded vibrational bands is observed extending from 675 nm to beyond 800 nm. These bands, constituting the most extensive vibrational structure observed to date for any of the rare gas dimers at small R, appear to be associated predominantly with a single upper electronic state. The proposed identity of this level, as well as the derived vibrational constants for the Rydberg Rydberg transitions, are discussed. The experimental approach implemented for these studies is based on the production of excited Xe2 molecules in the a3 Rþ u ð1u ; Ou Þ metastable states in a microplasma. After exploring several microdischarge structures and geometries, a simple, yet reliable and robust, design was found to consist of 1.5 mm diameter tungsten rods separated by 90 lm. Reproducible glow discharges were obtained in Xe at pressures of several hundred Torr by roughening the end faces of each rod and slightly rounding the edges in order to reduce field enhancement. These measures resulted in improved stability and ignition characteristics for the discharge which operated at a voltage and current density of 220 V and 226 mA cm2, respectively, for a Xe pressure of 400 Torr. Despite the 16:1 aspect ratio of the microplasma geometry, the discharge is a spatially uniform glow and is confined to the 160 nL volume between the electrodes [14]. For the data reported here (pXe = 400 Torr), the microplasma was crowbarred for 0.5 ls at a pulse repetition frequency of 10 Hz and, approximately 90 ns into
373
No laser pulse With laser
0.3
λdye=754.0 nm
0.25 0.2 0.15 0.1
Signal
0.05 0 -0.05
0
500
1000 1500 Time (ns)
2000
2500
Fig. 1. Waveforms, representative of those observed in these experiments, illustrating the suppression of Xe2 VUV emission by a dye laser pulse at kdye = 754.0 nm. The Xe pressure is 400 Torr and each waveform represents the average of 100 measurements.
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
the excited atomic fragment, will result in a fraction of the photoexcited molecules returning eventually to the a3 Rþ u state. Consider, for example, the ‘Signal’ waveform of Fig. 1 which represents the difference between the Xe2 fluorescence intensity traces recorded with, and without, the presence of the dye laser pulse. Roughly speaking, the early portion of the waveform corresponds to that time period in which population is removed from the Xe2 ða3 Rþ u Þ state, whereas a portion of the recovery of the VUV fluorescence and the negative signal evident beyond 550 ns reflect the return of a portion of that population. Obtaining reliable, unsaturated fluorescence suppression spectra requires that the variation of the degree of suppression with laser fluence be taken into account. Representative data, the results of 3150 measurements for the laser wavelength k fixed at 784.0 nm, are shown in Fig. 2. For the spectra reported here, the laser energy fluence delivered to the microplasma was fixed at 2 mJ cm2 (2.7 lJ per pulse for a plasma cross-section of 1.35 · 103 cm2) which is represented on the abscissa of Fig. 2 by a relative pulse energy of 0.4 and corresponds to a fluorescence suppression of 16 ±2%. The fluorescence suppression spectrum of Xe2 a3 Rþ u ð1u ; O u Þ, recorded in the 675–830 nm wavelength interval for a Xe pressure of 400 Torr, is indicated by the blue curve in Fig. 3. Each of the points comprising the spectrum is the average of 150 measurements at that wavelength. The overall spectrum has been corrected for the variation of the dye laser pulse energy with wavelength but has not as yet been normalized to the absolute excimer absorption cross-sections. The result (not illustrated in Fig. 3) is to flatten somewhat the spectral profile, particularly in the 12 500–13 000 cm1 region. A well-developed series of red-degraded vibrational bands is observed with individual bands exhibiting modulation depths as large as 60%. For comparison, a digitized segment of the absorption spectrum in 2310 Torr of Xe reported by Killeen and Eden in 1986 (Ref. [3]) is also given in Fig. 3 (red trace). It is immediately evident that all of the significant features in
0.4
Suppression
0.3 0.2 0.1 λ=784nm
0 − 0.1 0
0.5
1 2 1.5 Relative Pulse Energy
2.5
Fig. 2. Dependence of Xe2 fluorescence suppression on the relative dye laser pulse energy for a laser wavelength of 784 nm. Results are given for 3150 measurements at energy fluences, incident on the microplasma, up to 13 mJ cm2 per pulse.
3
14.5
-1
Wavenumbers (10 cm ) 14 13.5 13 12.5
12
0.22 0.2
Relative Suppression
374
0.18
0.3
Xe 400 Torr
Ref. 3
0.25
0.16
0.2
0.14 0.12
0.15
0.1 0.1
0.08 0.06
0.05 680
700
720 740 760 780 Wavelength (nm)
800
820
Fig. 3. Comparison of the laser-induced fluorescence suppression spec trum of Xe2 a3 Rþ u ð1u ; Ou Þ in the 675–830 nm region (blue curve) with the absorption spectrum reported in 1986 by Ref. [3] (in red). Each point in the suppression spectrum is the average of 150 measurements, and the overall spectrum is the composite of three segments, each of which required a separate laser dye.
the spectrum of Ref. [3] are matched by corresponding bands in the present data. One concludes that the ‘. . .distinct features at the peak of the near-infrared, quasicontinuum. . .’ observed in Ref. [3] are, indeed, real. Fewer than 10 vibrational bands lying between 730 and 770 nm were discernible in the electron beam experiments [3] whereas at least 20 distinct bands are detected in the present study. Relative to Ref. [3], the improved development of the vibrational structure reported here is attributable to several factors, including a significantly increased S/N ratio, a reduction in the Xe pressure by a factor of almost six, and a higher gas temperature. Analysis of the entire microplasma assembly, comprising the tungsten electrodes, ceramic (Macor) support structure, and the gas itself, with a one-dimensional thermal model predicts the average gas temperature in the microplasma to be 923 K for a steady-state current density of 0.23 A cm2 and a power input to the plasma of 880 mW (specific power loading of 5.5 kW cm3 to a plasma volume of 1.6 · 104 cm3). This situation contrasts with the low duty cycle, electron beam-driven absorption experiments of Ref. [3] in which the gas temperature did not rise far above ambient. Much of the structure in the experimental spectrum (Fig. 3) has been reproduced by simulations in which the upper and lower electronic states are represented by Morse potentials. Although the Morse function is known to be a poor representation of rare gas dimer Rydberg states at larger internuclear separations, itiis acceptable for a limited h 2 þ range in R near Re Xeþ X R . The observed transitions 1u 2 2
are assumed to terminate at a gerade state of Xe2 lying 12 500–13 500 cm1 above the a3 Rþ u ð1u ; Ou Þ metastable levels, and the vibrational constants for the lower state were taken to be those proposed by Jonin and Spiegelmann [2]
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377
for the 1(1u) state: x00e ¼ 109:0 cm1 and x00e x00e ¼ 1:14 cm 1 . More than 104 simulations were performed in which x0e ; x0e x0e ; and DR R0e R00e were iterated over the intervals ˚ , respectively. 80–140 cm1, 0.7–1.5 cm1, and 0.05–0.4 A For each set of spectroscopic constants, wavefunctions were calculated for both states and the Franck–Condon factors (FCFs) determined for v 0 , v00 6 20. Calculations ignored rotational effects but assumed the a3 Rþ u ð1u Þ vibrational population to be equilibrated and, thus, represented by a Boltzmann distribution for a temperature between 300 and 900 K. Of the 104 trials, 19 sets of constants provided an acceptable match to the experimental spectrum and the best fit yields the following constants for the upper state: x0e ¼122 3 cm1, x0e x0e ¼ 0:9 0:2 cm1, T 0e ¼ 12 980 ˚ , where the indicated 70 cm1, and DRe = 0.3 ± 0.1 A uncertainties represent one standard deviation determined from the 19 parameter sets. A comparison of the experimental and calculated spectra is presented in Fig. 4 for a gas temperature of 900 K. It is evident that the bandhead positions for the simulated spectrum, as well as the overall intensity profile at low photon energies (12 500– 13 000 cm1), match those of the experimental spectrum. The calculations have been less successful in reproducing weaker features and relative band strengths in the 13 100– 13 600 cm1 interval, but the abrupt loss of intensity for the vibrational band peaking at 14 010 cm1 is likely the result of a perturbation (cf. Fig. 4 and Ref. [2]). Also, a continuum appears to underlie the bound–bound structure in the short wavelength portion (13 700–14 800 cm1) of the experimental spectrum but has not thus far been incorporated into the simulations. The position and structure of the gerade Rydberg state of Xe2 determined from the simulations are also consistent with the predictions of pseudopotential hole-particle calculations of the dimer state structure reported in Ref. [2]. As noted earlier, Jonin and Spiegelmann have calculated the
375
interatomic potentials for both gerade and ungerade Rydberg states correlated, in the separated atom limit, with Xe (5p6 1S0)+ Xe (6s, 6p, 5d, 7s, 7p). Of particular relevance to the present spectroscopic measurements are the strongly bound Xe2 electronic states derived from Xe (1S0)+ Xe (6p). Gerade Rydberg states determined in Ref. [2] to lie 10 000–16 000 cm1 above T e ða3 Rþ u Þ of the dimer are illustrated in Fig. 5. Only two states, ð3ÞOþ g and 5(1g), are predicted to exist within ± 800 cm1 (± 0.1 eV) of the value of the upper state Te reported here (12 980 cm1). As discussed in Ref. [2], the constants for both states are expected to be similar, and Table 1 compares these theoretical values with those inferred from the experimental spectra. As will be discussed in more detail later, the dissociation energy D0e given in Table 1 is calculated by assuming Xe* (6p[5/2]2)+ Xe 5p6 1S0 to be the separated atom limit for the upper state associated with the observed transitions. Since our experiments directly yield only DRe, the value of R0e in ˚ , as proTable 1 is based upon taking R00e ð1u Þ to be 3.19 A posed in Ref. [2]. Given this assumption, the inferred value ˚ ) is in remarkable agreement with those of R0e (3.49 ± 0.1 A calculated by Jonin and Spiegelmann [2] for the candidate upper states ðð3ÞOþ g andð5Þ1g Þ mentioned earlier. In Ref. ˚ ) and 6.69a0 [2], R0e is determined to be 6.61a0 (3.50 A ˚ ) for the Oþ and 1g states, respectively, which reflect (3.54 A g the predominant contribution (70–75%) of the Xeþ 2 ðB2 P3=2g Þ state to the structure of the molecular ion core [15]. Dimer Rydberg states built on the ground state of the molecular ion X 2 Rþ are generally characterized by 1u 2 Re 6a0 (Ref. [2]). Of the other spectroscopic constants for the 3ðOþ g Þ and 5(1g) states calculated in Ref. [2], the anharmonic constant x0e x0e for 3ðOþ g Þ agrees with the experimental value to within the estimated uncertainty, whereas the theoretical values of Te, xe, and De for both states differ from the 20 5d
18
Energy (103 cm–1 )
6p
Relative Intensity
Experiment
Present Experiment
16
6p[5/2]2 6s
14 Ref. 16 (3)0g+ (5)1g
12 10
Simulation T=900 K
8
2
3
4
5
6
7
8
9
10
Internuclear Separation R(Å) 12000
12500
13000
13500
14000
14500
15000
Wavenumbers (cm−1) Fig. 4. (Top) Portion of the experimental suppression spectrum, smoothed relative to that of Fig. 2; (Bottom) Simulation of the spectrum for a gas temperature of 900 K. Most of the features in the experimental spectrum are tentatively assigned to 5ð1g Þ a3 Rþ u ð1u ; Ou Þ transitions of Xe2.
Fig. 5. Partial energy level diagram of Xe2 (adapted from Ref. [2]), indicating the location of the Rydberg state observed in the present experiments (red curve) as compared to the 5(1g) position calculated by Jonin and Spiegelmann (Ref. [2]). The extrapolated potential proposed by Dimov et al. (Ref. [16]) is also indicated (blue curve), and the ordinate (energy) is referenced to the value of Te for the 1ðO u Þ state of Xe2.
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Table 1 Spectroscopic constants for selected gerade states of Xe2, compared with values inferred from experimental spectra Spectroscopic constant ˚) Re (A Te (cm1)c xe (cm1) xexe (cm1) De (cm1)
Electronic state (3) Oþ g (5) 1g
Ref. [2] 3.50 3.54
(3) Oþ g (5) 1g
12395.0 12 388.0
(3) Oþ g (5) 1g
94.0 88.0
(3) Oþ g (5) 1g (3) Oþ g (5) 1g
1.18 1.26
Ref. [16]a
Present work 3.49 ± 0.1b
3.21 ± 0.03 12980 ± 70 11 756d 122 ± 3 128.6 ± 2.2 0.9 ± 0.2 1.587 ± 0.069 2250 ± 70d
2700
3470.4 ± 23.4
a Assumes that the constants reported in Ref. [16] are those for the (5)1g Rydberg state of Xe2. b ˚ (cf. Ref. [2]). Assumes R00e ¼3:19A c Relative to T e 1ðO u Þ — see Ref. [2]. d This value assumes Xe 6p[5/2]2 + Xe 5p6 1S0 to be the separated atom limit for the observed state.
experimental results by 5%, 30–40%, and [20%, respectively. It is clear, then, that the spectroscopic constants determined for the state responsible for the observed spectrum (Fig. 3) are quite close to those reported by Jonin and Spiegelmann for the 3ðOþ g Þ and 5(1g) states. However, differentiating between the Oþ g and 1g states as the upper level for the observed transitions is not possible at present and a definitive assignment will require the acquisition of rotationally-resolved spectra. In view of the strength of the observed transitions1 and the fact that ð3ÞOþ 1ðO g uÞ transitions are strictly forbidden, the assignment of 5ð1g Þ a3 R þ u ð1u ; Ou Þ is slightly favored. Nevertheless, this must be viewed as being tentative pending the availability of higher resolution spectra. We note that the choice of Xe 6p[5/2]2 + Xe 5p6 1S0 as the separated atom limit for the experimentally-observed state predicts the existence of a barrier in the potential, and a higher-lying asymptote would yield a value for De closer to the theoretical value. It should also be mentioned that, in 1994, Dimov et al. [16] observed a strongly bound dimerexcited state derived from Xe(5p6 1S0) and either Xe 6p 12 1 or 6p 52 2 . Resonantly-enhanced, multiphoton ionization (REMPI) experiments involving two photon excitation of the cooled dimer detected spectral features extending over a 350 cm1 wide region centered at 77 500 cm1, and tentatively assigned 0 to Oþ XOþ g g transitions terminating on 25 6 v 6 38 vibrational levels. Franck–Condon calculations suggested the upper state to be built on the ground state molecular ion X2 Rþ core, and the Dunham coefficients were deter1u 2 mined to be T 0e + Y0,0 = 74 846.15 ± 23.2 cm1, Y1,0
ð x0e Þ ¼ 128:6 2:2 cm1, Y2,0 ð x0e x0e Þ ¼ 1:587 0.069 cm1, and D0e ¼ 3470:4 23.4 cm1. Since only the high v 0 (P25) portion of the interaction potential was observed, extrapolation to the region near the potential minimum ðR R0e Þ was necessary, and the result reported in Ref. [16] is compared in Fig. 5 with the potential derived from the spectra reported here. Both interatomic potentials are portrayed by the Morse function and the solid and dashed portions of the red curve (present work) represent the experimentally-observed and extrapolated regions, respectively. Although the state reported by Dimov et al. [16] lies close in energy to the Rydberg level observed in the present experiments and a portion of the discrepancies between the two sets of constants in Table 1 are attribut able to the uncertainty in T e a3 Rþ u ð1u ; Ou Þ and the extrapolation over v 0 required in Ref. [16] to estimate spectroscopic parameters, the substantial differences in R0e and D0e suggest that the REMPI experiments observed a state distinct from that reported here. In summary, red-degraded vibrational bands associated with Rydberg Rydberg state absorptive transitions of Xe2 have been observed in the 675–800 nm region by laser-induced fluorescence suppression spectroscopy. Tentatively assigned to 5ð1g Þ a3 R þ u ð1u Þ transitions of the dimer, the observed spectrum exhibits the most extensive vibrational structure for anyiof the rare gas observed h 2 þ dimers at small R R ffi Re Xeþ ðX R1u Þ . This is a result 2 2 of the gas temperature in the plasma (900 K) and the ˚ ) between the equilibrium internuclear large offset (0.3 A separations (Re) for the two electronic states responsible for the spectrum. Simulations of the experimental spectra yield spectroscopic constants that are consistent with pseudopotential hole-particle theory. Acknowledgements The technical assistance of K. Collier and N. Ostrom, as well as the support of the US Air Force Office of Scientific Research under Grant No. F49620-03-1-0391, are gratefully acknowledged. References [1] [2] [3] [4]
[5] [6] [7] [8]
1 By measuring the dependence of the fluorescence suppression on laser intensity at several laser probe wavelengths, the absorption cross-sections for the Rydberg a3 Rþ u ð1u ; Ou Þ transitions have been measured to be 1017 cm2. These experiments will be described in detail elsewhere.
[9] [10]
M.L. Ginter, J.G. Eden, Can. J. Chem. 82 (2004) 762. C. Jonin, F. Spiegelmann, J. Chem. Phys. 117 (2002) 3059. K.P. Killeen, J.G. Eden, J. Chem. Phys. 84 (1986) 6048. The first indications of this band were observed by E. Zamir, D.L. Huestis, H.H. Nakano, R.M. Hill, D.C. Lorents, IEEE J. Quantum Electron. 15 (1979) 281. C. Jonin, P. Laporte, R. Saoudi, J. Chem. Phys. 108 (1998) 480. C. Jonin, P. Laporte, F. Spiegelmann, Chem. Phys. Lett. 308 (1999) 13. C. Jonin, E. Descroix, P. Laporte, F. Spiegelmann, Chem. Phys. Lett. 300 (2000) 595. S. Arai, T. Oka, M. Kogoma, M. Imamura, J. Chem. Phys. 68 (1978) 4595. M. Hemici, R. Saoudi, E. Descroix, E. Audouard, P. Laporte, F. Spiegelmann, Phys. Rev. A 51 (1995) 3351. A.W. McCown, M.N. Ediger, S.M. Stazak, J.G. Eden, Phys. Rev. A 28 (1983) 1440.
C.J. Wagner, J.G. Eden / Chemical Physics Letters 422 (2006) 372–377 [11] S.B. Kim, D.J. Kane, J.G. Eden, Phys. Rev. Lett. 68 (1992) 1311. [12] A.W. McCown, M.N. Ediger, J.G. Eden, Phys. Rev. A 28 (1983) 3365. [13] A.W. McCown, M.N. Ediger, D.B. Geohegan, J.G. Eden, J. Chem. Phys. 82 (1985) 4862; D.B. Geohegan, J.G. Eden, Chem. Phys. Lett. 139 (1987) 519.
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[14] C.J. Wagner, J.G. Eden, IEEE Trans. Plasma Sci. 33 (2005) 568. [15] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [16] S.S. Dimov, J.Y. Cai, R.H. Lipson, J. Chem. Phys. 101 (1994) 10313.