- Email: [email protected]
Vacuum ultraviolet laser spectroscopy of Xe2 between 116.9 and 123.6 nm
JOURNALOFMOLECULAR
SPECTROSCOPY
(1992)
151,312-321
Vacuum Ultraviolet Laser Spectroscopy of Xen between 116.9 and 123.6 nm KOICHI TSURIYAMA RIKEN,
AND
TAKAHIRO
KASUYA
The Institute of Physical and Chemical Research, Hirosawa, Wako, Saitama 351-01, Japan
Vibrationally resolved fluorescence excitation spectra of jet-cooled Xe2 have been recorded between 116.9 and 123.6 nm with tunable coherent radiation generated by four-wave mixing in Kr. Three molecular systems were observed. One is a new band system in the vicinity of the i19.2-nm Xe resonance line. The other two are system X and XI which have been identified by absorption spectroscopy [M. C. Castex, Chem. Phys. 5, 448-455 ( 1974)]. The analyses of the latter two band systems provided accurate spectroscopic constants of the upper states for the first time. The new band and X band systems were shown to be predissociated to the ground ( ‘SC,) and the excited state ( 5ps6p) Xe atoms. 0 1992Academic press. hc. I. INTRODUCTION
The excited states of Xez have been the subject of extensive experimental and theoretical investigations because the excimer transition is a possible source of intense vacuum ultraviolet (VUV) laser radiation. Castex and Damany (l-3) and Freeman et al. (4) mapped out the ground and several ungerade excited states in their early spectroscopic studies. The analysis of the spectra, however, was not satisfactory due to poor resolution of VUV light sources and pressure broadening of atomic resonance lines. These problems were practically solved by the use of ( 1) tunable VUV coherent radiation generated by the four-wave mixing process and (2) a pulsed supersonic jet to form rotationally and vibrationally cold ground state Xez in almost collision-free conditions (5- 7). Thus far, unambiguous vibrational quantum numbers and precise molecular constants have been determined for the four lowest ungerade excited states, namely, the A 1u state correlating to Xe ( ’ So) + Xe* 6s[ &, BOu+ to 6s[ $1I , and COu+ and C’lu to 6&i. In this paper, we present information about the generation of tunable coherent XUV and VUV radiation via the Sp’[$, level of Kr and also the laser-induced fluorescence study of Xez in the region between 116.9 and 123.6 nm. Three molecular systems have been recorded. Among them was a new band system located near the 119.2-nm (Xe* 5 d[ ;], ) atomic line. The other two systems correspond to systems X and XI which have been identified by absorption spectroscopy (4). The analyses of the vibrationally resolved excitation spectra provided accurate molecular parameters of the upper states for the first time. II. EXPERIMENTAL
DETAILS
Continuously tunable coherent radiation between 116.9 and 123.6 nm was generated by resonant four-wave mixing in Kr. The sum and difference frequency mixing via the 5p levels has been investigated in detail (8, 9). The two-photon resonant intermediate state employed in the present study was the Sp’[$]o level (E = 98 855.871 0022-2852192 $3.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
312
313
SPECTROSCOPY OF Xez
cm-r). The excitation of this level requires two UV (202.3 1 nm) photons which can be generated efficiently by the frequency tripling technique (IO). The 606.93 nm output of the dye laser I (Quanta Ray PDL-2, Rh-640 in methanol) was frequency doubled in a KDP crystal (Lambda Physik, 7 X 14 X 30 mm, cutting angle 62”). The second harmonic (303.46 nm) was separated from the residual fundamental light by a dichroic mirror. A double Fresnel rhomb was used to rotate the polarization of the visible light to match that of the second harmonic. The beams were collinearly superposed through a dichroic mirror and directed to a BBO crystal (Quantum Technology, 5 X 5 X 6.5 mm, cutting angle 50.2”). The resulting 202.31-nm light, separated from the visible and the second harmonic by two dichroic mirrors, was collinearly coupled with the output of the dye laser II (Quanta Ray PDL-2). Laser beams were then focused by a fused silica lens (f = 25 cm) into a mixing chamber where Kr gas was provided as a pulsed jet. Stainless steel parallel plates were set at - 1 cm downstream of a pulsed valve (Laser Technics LPV, nozzle diameter = 0.5 mm) to detect Kr ions produced by the (2 + 1) REMPI of 202.3 1 nm photon. This procedure serves as a simple check of the resonant condition. A part of the tuning characteristics is illustrated in Fig. 1. By the use of several commercial rhodamine and LDS dyes (550-750 nm), 85.4-89.1- and 116.9-123.9nm spectral regions were covered by sum and difference frequency mixing, respectively. In the XUV region, the conversion efficiency is significantly enhanced in the vicinity of the auto-ionizing s’ and d’ levels as shown in Fig. 1(a). On the other hand, in the VUV region, the tuning curve is smooth except for a dip (see Fig. 1(b)). Since the profile of this dip is similar to the enhancement pattern in the corresponding XUV
123.5
124.0
123.0 (nm)
I’
17600
16000
18200
85.5
17600 85.9
85.1
lld' I
10d' I
13s' I
9d' 1
125' ,
fl
ci ii, 4’
(nm)
115' /
E
!I /f+y \
;’
p
i
/ I
18200
18000
LASER
i 17600
WAVENUMBER
17600
(c rn-‘)
FIG. 1. Tuning characteristics of the coherent radiation generated by four-wave difference (upper trace) and sum (lower trace) frequency mixing in Kr.
314
TSUKIYAMA AND KASUYA
region, the observed dip might be induced by the competition between sum and difference frequency generation. The conversion efficiency below 123.6 nm (Kr* 6s’[ 411) is higher by one order of magnitude than above. The generated XUV/VUV radiation was introduced through a skimmer of 5 mm diameter into a main chamber where Xez was formed in a pulsed jet of neat Xe. A stagnation pressure of typically 1 atm provided the vibrationally cold Xez . The pressure of the main chamber was kept below 8 X 10e4 Torr by the use of 6” oil diffusion and a mechanical booster pump. Since the photon energy of the XUV radiation exceeds the ionization threshold of Xez, there is little doubt that the fluorescence observed in the present study was excited by the VUV radiation. The VUV fluorescence was detected by a solar blind photomultiplier (Hamamatsu R1459 or R1259) through a MgFz (Xobs.3 115 nm), a CaFz (Xobs,3 123 nm), or a sapphire ( Xob, 3 142 nm) window, whereas the near-IR fluorescence was observed by a photomultiplier (Hamamatsu R-666) with appropriate filters (HOYA IR-80 or 85 ) . The excitation spectrum was recorded with a boxcar integrator (Stanford SR-250). The wavenumber of the dye laser II was calibrated by simultaneously recording the excitation spectrum of I2 or the laser transmission through a Fabry-Perot etalon (Burleigh TL-38; FSR = 2.548 cm-’ ) ( 11). The wavenumbers calibrated by the above two methods agreed within +0.3 cm-’ _ The deviation is mainly due to the nonlinearity of the scanning mechanism of the dye laser. The resolution (FWHM) of the dye laser output was 0.11 and 0.25 cm-’ with and without an intracavity &talon, respectively. The resolution (FWHM) of the VUV radiation, as estimated from the line width of the Xe resonance line, was -0.9 cm-’ without the intracavity &talon. III. RESULTS AND DISCUSSION
III-A.
The New Band System
The fluorescence excitation spectrum of Xe2 near the Xe* 5 d[$ll level (E = 83 890.47 cm-‘) is shown in Fig. 2 where VUV fluorescence ranging from 115 to 190 nm was monitored; the band positions are given in Table I. Because of the long progression in n’, the lowest observed 2)’level is assigned 2)’= m instead of 21’= 0. Two features should be noted: ( 1) The vibrational spacings increase slightly with 21’and (2) peaks split at u’ = m + 4 and m + 5. Lipson et al. (6) have used the isotope
I
FIG.
line.
Xe 5d [3/Z],
2. Fluorescence excitation specrum of the new band system in the vicinity of 119.2 nm Xe resonance
315
SPECTROSCOPY OF Xel TABLE I The New Band System transitIon
V’
wavenumber(cm-‘1
unasslnned
83691
unasslened
83743.?
m m+
.8
83761. 1
=
51..1
cm-’
_l
=
:i:1.0
cm-1
1
83794.4
In+2
83828.5
m+3
83862.9
m+4
X3&95. 839U
.”
83933. 83941
9 .G
m+5
A
3 I
1
.! ,I
4
H
structure of Xez to determine the absolute numbering of 2)‘.The isotope shift is smallest for 2)’= 0 and increases with vibrational quantum number, thus may not be resolvable for low vibrational quantum numbers. In fact, the vibrational levels identified by Lipson et al. (6) have large vibrational quantum numbers (0’ 3 19). The absence of resolved isotope structure in the present spectrum indicates that the vibrational quantum number is not as high as theirs. The peaks at low quantum numbers are degraded to the blue as a result of unresolved rotational structure and consequently the equilibrium internuclear distance of the upper state would be shorter than that (4.342 A) of the ground state. Assuming that the upper state correlates to the Xe ( ‘So) + Xe* 5 d[ i], level, a lower limit of the dissociation energy DOis - 3 15 cm-’ . Two absorption band systems in the vicinity of the Xe* 5 d[ f], level have been recorded by Castex and Damany (I, 2), systems III (83 350-83 700 cm-‘) and IV (83 950-84 330 cm-‘), which are located on the red and blue sides of the resonance line, respectively. We could not detect these two systems under our experimental conditions. Since the vibrational spacing (33 cm-‘) of the present system is much smaller than that (55 - 75 cm-‘) of system III, it is unlikely that the present system consists of the higher vibrational members of system III. On the other hand, the spacing ( -52 cm-‘) of the two unassigned broad peaks listed in Table I coincides with that ( -55 cm-‘) of system III near its high-frequency end. The possibility cannot be ruled out that these two unassigned peaks belong to system III. It is interesting to note that the excitation of the upper state results in the fluorescence in the near-IR region as well as in VUV. Figures 3 (a) and (b) illustrate excitation spectra where VUV fluorescence above 115 and 123 nm was monitored, respectively. Since the dissociation energy of the ground state is - 195 cm-‘, VUV fluorescence above 123 nm cannot originate from the bound-bound transition between the upper and the ground state. The same excitation spectrum as in Fig. 3 (b) was obtained with a sapphire window. The spectral trace in Fig. 3 (c), where near-IR fluorescence (filter in use: IR-80, 800 G Xobs.9 9 10 nm) was monitored, is identical with that in 3 (b). The use of IR-85 (850 s Xobs.G 9 10 nm) reduced the signal intensity by a factor of about 4, indicating that the major part of the near-IR fluorescence falls in the region between 800 and 850 nm.
316
TSUKIYAMA AND KASUYA
a)
b)
cl
Il.&L &!=&l_ 83800
8390
FIG. 3. Excitation spectra of the new band system: (a) 115 C X,,. G 190 nm, (b) 122 G X,+. G 190 nm, (c) 800 G Xoba.G 900 nm.
These observations are well explainable by the predissociation of the upper state; the upper bound state intersects the repulsive curve dissociating to Xe ( I&) and Xe* ( 5g56p) states. The latter manifold emits near-IR photons to produce the Xe* 6s[ $I1 level which subsequently fluoresces at 147.0 nm. As listed in Table II, 6p[&, [ $12, and [ $1, levels are responsible for the near-IR emission ( 12). It should be noted that the ratio of the fluorescence intensity at u’ = m to m + 1 is much smaller in Figs. 3 (b) and (c) than in (a) f suggesting that the radiative process and the predissociation are competing at 2)’ = m. The repulsive curve might cross the upper bound state between 2)’= m and m + 1. The alternative explanation is that the repulsive curve associates with the Xe* ( 5p55 d) levels which undergo radiative cascading (Xe 5 d --, 6p * 6s). This mechanism, however, would be of minor importance because radiative lifetimes of the Xe* ( 5p55 d) manifold are long (several ps) whereas there is no such slow component in the risetime of the VUV fluorescence. Our experiment reveals that at least three molecular systems, namely system III, system IV, and the new band system, are located near the Xe* 5 d[i]i level. The
317
SPECTROSCOPY OF Xe2 TABLE II
Summary of Term Energies, Branching Ratios, and Emission Wavelengths for Xe* ( 5p56p) States
notation
energy
final
state
branching
(cm-l,
wavelength
ratio
tnm>
6~[1/210
80119.5
6s[3/211
0.998
828.0
6p[3/212
79213.0
6s[3/212
0.700
823.2
6s[3/211
0.300
895.2
6s[3/212
0.084
840.9
6s[3/21,
0.915
916.3
6p[3/211
78956.5
combination of the two atomic terms 5p6 ’ SOand 5 d[ $1r gives rise to two ungerade states, OU+and 1U. Castex and Damany ( 1, 2) tentatively assigned systems III and IV to OU+ and lu, respectively. If one assumes that system IV associates to the 5 d[$, level, a potential hump of -200 cm-’ above the asymptote must be taken into consideration. All vibrational levels involved in the new molecular system are located below the 5d[i], dissociation limit, and consequently it seems more reasonable to ascribe the present system to one of the upper states associated with 5 d[ 21, level. The ab initio calculation might aid in establishing definitive assignments of these excited states.
According to Castex (3), system X is composed of two broad bands about 280 cm-’ apart (left column of Table III). In the present study, poor S/N ratio hampered
TABLE III The System X cas texa
This
transition wavenumber
V’ (cm-l)
work
transition wavenumber
(cm-‘)
82719.2 ‘)
82720.3b
82442.3
0
82445.8
82435.5
1
02457.7
82708.2
A
=
11.9
a Ref. (3). h This peak has a broad ( -5 cm-‘) feature superposed on a continuum.
cm-’
318
TSUKIYAMA AND KASUYA
a)
SYSTEM
v’=O
I
1 I
X
I
h
b)
82!4 00
82450
82500
cm-’ FIG. 4. Ruorescence excitation specrum of the system X: (a) 115 5 k,&. g 190 nm, (b) 800 G X0,. g 900 nm.
the detailed analysis of the band on the high-frequency side; the fluorescence intensity is one order of magnitude weaker than the new band system described in the previous section. The peak, superposed on a continuum, has a broad ( -5 cm-‘) feature. The band system on the low-frequency side shows a resolved vibrational structure as illustrated in Fig. 4. Both VUV (X 3 142 nm) and near-IR (800 S X S 900 nm) fluorescence were observed with the excitation of this band, suggesting that the upper state is predissociative. Transition wavenumbers are tabulated in Table III along with their vibrational numberings. A weak peak at 82 437.1 cm-’ is ascribed to a hot band (u’ = 1 + v” = 1) because the estimated vibrational wavenumber ( - 19.4 cm-’ ) of the ground state is in good agreement with the literature value of 19.9 cm-’ (4). The present system must relate to the Xe* 5 d[$ level since this level is the only one in the neighborhood The dissociation energy Do of the upper state is then calculated to be 170.1 cm-’ which is somewhat shallower than that of the ground state ( 185.2 cm-’ ) (4). The following values are derived: wk = 12.33 cm-‘, w,xk = 0.22 cm-‘, and De = 176.2 cm-‘. The Morse estimate of D, ( = wz/4o,x, + 173 cm-‘) is fairly consistent with the above value.
319
SPECTROSCOPY OF Xez
a)
v’
q
0 r
1
2
3
I
I
, SYSTEM
XI
b)
, 000
I 85100
I
1
#
85200 cm-'
FIG. 5. (a) Fluorescence excitation specrum of the system XI. Intensity of the VUV coherent radiation is almost constant over the entire region. (b) Calculated Franck-Condon factors: r: = 4.52 A.
Calculation of the Franck-Condon factors’ was carried out to evaluate the internuclear distance (rb) of the upper state. Morse potential curves were assumed for both ground and excited states. The molecular parameters for the ground state were fixed to the following values: w[ = 2 1.07 cm-‘, w,x: = 0.63 cm-l, and r Z = 4.362 A. The rotational constant B: of the excited state was varied until a reasonable agreement between observed and calculated intensities was achieved. For the upper state of system X, however, it was not possible to determine whether rk is greater or smaller than the ’ We have used the version from Dr. Jon T. Hougen of the National Institute of Standards and Technology for the RKR and Franck-Condon factor program which was originally written by Zare. Schmeltekopf. Harrop, and Albritton for use in their article in J. Mol. Spectrosc. 46, 37-66 ( 1973).
320
TSUKIYAMA
AND KASUYA
TABLE IV The System XI V’
V”
0
1
85047.2
0
0
85066.8
1
0
85091.0
2
0
85114.6
3
0
85137.7
transition
(cm-‘)
wavenumber
A
=
24.2
cm-l
23.6
23.1
ground state value, because in both cases the intensity of 2)’= 2 was calculated to be much smaller than that of 2)’= 1. To the best fit, r’, is 0.09 - 0.10 A apart from the ground state. The system IX around 82 000 cm-’ identified by absorption spectroscopy (4) could not be detected under our experimental conditions. This might reflect the magnitude of the absorption strength: the pressure of system IX was -40 Torr, whereas that of system X was only 5 Torr (3). Ill-C.The System XI
The system XI was described as an intense molecular system appearing at a pressure of3 - 6 Torr ( 3)) and was observed with a favorable S/N ratio as reproduced in Fig. 5 (a). Transition wavenumbers are tabulated in Table IV along with their vibrational numberings. As the progressions have the greatest intensity for the lowest u’, the most intense peak was assigned 21’= 0. The following spectroscopic constants are derived by the least-square method: ok = 24.8 cm-’ and w:x: = 0.28 cm-‘. Two nearby atomic levels, namely, 7s[$ (E = 85 189.31 cm-‘) and 7s[& (E = 85 440.53 cm-‘), could be proposed as the dissociation limit. The Do evaluated for these levels are 307.7 and 558.9 cm-i, respectively. The Morse estimate of Do (- 560 cm-’ ) favors the latter assignment. This system fluoresces only in the VUV region below 122 nm, indicating that the bound-bound transition between the upper and the ground state is dominant. We found that the intensity distributions were fairly consistent with FCF calculations with r: = 4.52 A as shown in Fig. 5(b). An attempt to fit with smaller internuclear distance than the ground state yielded less satisfactory results; the intensities of IJ’ = 2 and 3 were calculated to be about one and two orders of magnitude different from that of 2)’ = 1. In addition, (2)’ = 1 and 2) + v” = 1 transitions were estimated to have larger FCFs than the ~1’= 0 + 2)”= 1 transition. No significant difference was found by the adoption of Rydberg-Klein-Rees (RKR) potentials. In conclusion, laser-induced fluorescence study of jet-cooled Xez with spectrally strong, monochromatic VUV coherent radiation resulted in ( 1) the first observation of the molecular system in the vicinity of the 119.2~nm Xe atomic line and (2) the first accurate determination of the spectroscopic constants of systems X and XI. The symmetry of the upper states, however, could not be determined from the present
SPECTROSCOPY OF Xe2
321
study. The calculation of the potential curves correlating to Xe* ( 5p56p, 5p55 d and 5~~7s) is highly desirable for the better characterization of the excited states. IV. ACKNOWLEDGEMENT This work was supportedin part by the Kurata Foundation. The authors thank Dr. T. Suzuki for the loan of the Fabry-Perot etalon system and Dr. T. Ishiwata and Dr. T. Munakata for their instructions concerning Franck-Condon factor calculations. RECEIVED:
July 30, 199 1 REFERENCES
1. M. C. CASTEXAND N. DAMANY, Chem. Phys. Lett. 24,437-440 ( 1974). 2. M. C. CASTEXAND N. DAMANY, Chem. Phys. Lett. 13, 158-161 (1972). 3. M. C. CASTEX,Chem. Phys. 5,448-455 (1974). 4. D. E. FREEMAN,K. YOSHINO,AND Y. TANAKA, J. Chem. Phys. 61,4880-4889 ( 1974). 5. R. H. LIPSON,P. E. LAROCQUE,AND B. P. STOICHEFF, Opt.Lett. 9,402-404 (1984). 6. R. H. LIPSON,P. E. LAROCQUE,AND B. P. STOICHEFF, .I. Chem. Phys. 82,4470-4478 ( 1985). 7. K. TSUKIYAMA,M. TSUKAKOSHI,AND T. KASUYA, Chem. Phys. 127,393-397 ( 1988). 8. K. D. BONIN AND T. J. MCILRATH, J. Opt.Sm. Am. B 2, 527-534 ( 1988). 9. G. HILBER,A. LAGO, AND R. WALLENSTEIN,J. Opt. Sot. Am. B 4, 1753-1764 ( 1987). 10. W. L. GLAB AND J. P. HESSLER, Appi. Opt. 26, 3181-3182 (1987). 11. T. SUZUKIAND T. KASUYA, Phys. Rev. A 36,2129-2133 (1987). 12. N. BOWERING,M. R. BRUCE,AND J. W. KETO, J. Chem. Phys. 84,709-714 (1986).
SPECTROSCOPY
(1992)
151,312-321
Vacuum Ultraviolet Laser Spectroscopy of Xen between 116.9 and 123.6 nm KOICHI TSURIYAMA RIKEN,
AND
TAKAHIRO
KASUYA
The Institute of Physical and Chemical Research, Hirosawa, Wako, Saitama 351-01, Japan
Vibrationally resolved fluorescence excitation spectra of jet-cooled Xe2 have been recorded between 116.9 and 123.6 nm with tunable coherent radiation generated by four-wave mixing in Kr. Three molecular systems were observed. One is a new band system in the vicinity of the i19.2-nm Xe resonance line. The other two are system X and XI which have been identified by absorption spectroscopy [M. C. Castex, Chem. Phys. 5, 448-455 ( 1974)]. The analyses of the latter two band systems provided accurate spectroscopic constants of the upper states for the first time. The new band and X band systems were shown to be predissociated to the ground ( ‘SC,) and the excited state ( 5ps6p) Xe atoms. 0 1992Academic press. hc. I. INTRODUCTION
The excited states of Xez have been the subject of extensive experimental and theoretical investigations because the excimer transition is a possible source of intense vacuum ultraviolet (VUV) laser radiation. Castex and Damany (l-3) and Freeman et al. (4) mapped out the ground and several ungerade excited states in their early spectroscopic studies. The analysis of the spectra, however, was not satisfactory due to poor resolution of VUV light sources and pressure broadening of atomic resonance lines. These problems were practically solved by the use of ( 1) tunable VUV coherent radiation generated by the four-wave mixing process and (2) a pulsed supersonic jet to form rotationally and vibrationally cold ground state Xez in almost collision-free conditions (5- 7). Thus far, unambiguous vibrational quantum numbers and precise molecular constants have been determined for the four lowest ungerade excited states, namely, the A 1u state correlating to Xe ( ’ So) + Xe* 6s[ &, BOu+ to 6s[ $1I , and COu+ and C’lu to 6&i. In this paper, we present information about the generation of tunable coherent XUV and VUV radiation via the Sp’[$, level of Kr and also the laser-induced fluorescence study of Xez in the region between 116.9 and 123.6 nm. Three molecular systems have been recorded. Among them was a new band system located near the 119.2-nm (Xe* 5 d[ ;], ) atomic line. The other two systems correspond to systems X and XI which have been identified by absorption spectroscopy (4). The analyses of the vibrationally resolved excitation spectra provided accurate molecular parameters of the upper states for the first time. II. EXPERIMENTAL
DETAILS
Continuously tunable coherent radiation between 116.9 and 123.6 nm was generated by resonant four-wave mixing in Kr. The sum and difference frequency mixing via the 5p levels has been investigated in detail (8, 9). The two-photon resonant intermediate state employed in the present study was the Sp’[$]o level (E = 98 855.871 0022-2852192 $3.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
312
313
SPECTROSCOPY OF Xez
cm-r). The excitation of this level requires two UV (202.3 1 nm) photons which can be generated efficiently by the frequency tripling technique (IO). The 606.93 nm output of the dye laser I (Quanta Ray PDL-2, Rh-640 in methanol) was frequency doubled in a KDP crystal (Lambda Physik, 7 X 14 X 30 mm, cutting angle 62”). The second harmonic (303.46 nm) was separated from the residual fundamental light by a dichroic mirror. A double Fresnel rhomb was used to rotate the polarization of the visible light to match that of the second harmonic. The beams were collinearly superposed through a dichroic mirror and directed to a BBO crystal (Quantum Technology, 5 X 5 X 6.5 mm, cutting angle 50.2”). The resulting 202.31-nm light, separated from the visible and the second harmonic by two dichroic mirrors, was collinearly coupled with the output of the dye laser II (Quanta Ray PDL-2). Laser beams were then focused by a fused silica lens (f = 25 cm) into a mixing chamber where Kr gas was provided as a pulsed jet. Stainless steel parallel plates were set at - 1 cm downstream of a pulsed valve (Laser Technics LPV, nozzle diameter = 0.5 mm) to detect Kr ions produced by the (2 + 1) REMPI of 202.3 1 nm photon. This procedure serves as a simple check of the resonant condition. A part of the tuning characteristics is illustrated in Fig. 1. By the use of several commercial rhodamine and LDS dyes (550-750 nm), 85.4-89.1- and 116.9-123.9nm spectral regions were covered by sum and difference frequency mixing, respectively. In the XUV region, the conversion efficiency is significantly enhanced in the vicinity of the auto-ionizing s’ and d’ levels as shown in Fig. 1(a). On the other hand, in the VUV region, the tuning curve is smooth except for a dip (see Fig. 1(b)). Since the profile of this dip is similar to the enhancement pattern in the corresponding XUV
123.5
124.0
123.0 (nm)
I’
17600
16000
18200
85.5
17600 85.9
85.1
lld' I
10d' I
13s' I
9d' 1
125' ,
fl
ci ii, 4’
(nm)
115' /
E
!I /f+y \
;’
p
i
/ I
18200
18000
LASER
i 17600
WAVENUMBER
17600
(c rn-‘)
FIG. 1. Tuning characteristics of the coherent radiation generated by four-wave difference (upper trace) and sum (lower trace) frequency mixing in Kr.
314
TSUKIYAMA AND KASUYA
region, the observed dip might be induced by the competition between sum and difference frequency generation. The conversion efficiency below 123.6 nm (Kr* 6s’[ 411) is higher by one order of magnitude than above. The generated XUV/VUV radiation was introduced through a skimmer of 5 mm diameter into a main chamber where Xez was formed in a pulsed jet of neat Xe. A stagnation pressure of typically 1 atm provided the vibrationally cold Xez . The pressure of the main chamber was kept below 8 X 10e4 Torr by the use of 6” oil diffusion and a mechanical booster pump. Since the photon energy of the XUV radiation exceeds the ionization threshold of Xez, there is little doubt that the fluorescence observed in the present study was excited by the VUV radiation. The VUV fluorescence was detected by a solar blind photomultiplier (Hamamatsu R1459 or R1259) through a MgFz (Xobs.3 115 nm), a CaFz (Xobs,3 123 nm), or a sapphire ( Xob, 3 142 nm) window, whereas the near-IR fluorescence was observed by a photomultiplier (Hamamatsu R-666) with appropriate filters (HOYA IR-80 or 85 ) . The excitation spectrum was recorded with a boxcar integrator (Stanford SR-250). The wavenumber of the dye laser II was calibrated by simultaneously recording the excitation spectrum of I2 or the laser transmission through a Fabry-Perot etalon (Burleigh TL-38; FSR = 2.548 cm-’ ) ( 11). The wavenumbers calibrated by the above two methods agreed within +0.3 cm-’ _ The deviation is mainly due to the nonlinearity of the scanning mechanism of the dye laser. The resolution (FWHM) of the dye laser output was 0.11 and 0.25 cm-’ with and without an intracavity &talon, respectively. The resolution (FWHM) of the VUV radiation, as estimated from the line width of the Xe resonance line, was -0.9 cm-’ without the intracavity &talon. III. RESULTS AND DISCUSSION
III-A.
The New Band System
The fluorescence excitation spectrum of Xe2 near the Xe* 5 d[$ll level (E = 83 890.47 cm-‘) is shown in Fig. 2 where VUV fluorescence ranging from 115 to 190 nm was monitored; the band positions are given in Table I. Because of the long progression in n’, the lowest observed 2)’level is assigned 2)’= m instead of 21’= 0. Two features should be noted: ( 1) The vibrational spacings increase slightly with 21’and (2) peaks split at u’ = m + 4 and m + 5. Lipson et al. (6) have used the isotope
I
FIG.
line.
Xe 5d [3/Z],
2. Fluorescence excitation specrum of the new band system in the vicinity of 119.2 nm Xe resonance
315
SPECTROSCOPY OF Xel TABLE I The New Band System transitIon
V’
wavenumber(cm-‘1
unasslnned
83691
unasslened
83743.?
m m+
.8
83761. 1
=
51..1
cm-’
_l
=
:i:1.0
cm-1
1
83794.4
In+2
83828.5
m+3
83862.9
m+4
X3&95. 839U
.”
83933. 83941
9 .G
m+5
A
3 I
1
.! ,I
4
H
structure of Xez to determine the absolute numbering of 2)‘.The isotope shift is smallest for 2)’= 0 and increases with vibrational quantum number, thus may not be resolvable for low vibrational quantum numbers. In fact, the vibrational levels identified by Lipson et al. (6) have large vibrational quantum numbers (0’ 3 19). The absence of resolved isotope structure in the present spectrum indicates that the vibrational quantum number is not as high as theirs. The peaks at low quantum numbers are degraded to the blue as a result of unresolved rotational structure and consequently the equilibrium internuclear distance of the upper state would be shorter than that (4.342 A) of the ground state. Assuming that the upper state correlates to the Xe ( ‘So) + Xe* 5 d[ i], level, a lower limit of the dissociation energy DOis - 3 15 cm-’ . Two absorption band systems in the vicinity of the Xe* 5 d[ f], level have been recorded by Castex and Damany (I, 2), systems III (83 350-83 700 cm-‘) and IV (83 950-84 330 cm-‘), which are located on the red and blue sides of the resonance line, respectively. We could not detect these two systems under our experimental conditions. Since the vibrational spacing (33 cm-‘) of the present system is much smaller than that (55 - 75 cm-‘) of system III, it is unlikely that the present system consists of the higher vibrational members of system III. On the other hand, the spacing ( -52 cm-‘) of the two unassigned broad peaks listed in Table I coincides with that ( -55 cm-‘) of system III near its high-frequency end. The possibility cannot be ruled out that these two unassigned peaks belong to system III. It is interesting to note that the excitation of the upper state results in the fluorescence in the near-IR region as well as in VUV. Figures 3 (a) and (b) illustrate excitation spectra where VUV fluorescence above 115 and 123 nm was monitored, respectively. Since the dissociation energy of the ground state is - 195 cm-‘, VUV fluorescence above 123 nm cannot originate from the bound-bound transition between the upper and the ground state. The same excitation spectrum as in Fig. 3 (b) was obtained with a sapphire window. The spectral trace in Fig. 3 (c), where near-IR fluorescence (filter in use: IR-80, 800 G Xobs.9 9 10 nm) was monitored, is identical with that in 3 (b). The use of IR-85 (850 s Xobs.G 9 10 nm) reduced the signal intensity by a factor of about 4, indicating that the major part of the near-IR fluorescence falls in the region between 800 and 850 nm.
316
TSUKIYAMA AND KASUYA
a)
b)
cl
Il.&L &!=&l_ 83800
8390
FIG. 3. Excitation spectra of the new band system: (a) 115 C X,,. G 190 nm, (b) 122 G X,+. G 190 nm, (c) 800 G Xoba.G 900 nm.
These observations are well explainable by the predissociation of the upper state; the upper bound state intersects the repulsive curve dissociating to Xe ( I&) and Xe* ( 5g56p) states. The latter manifold emits near-IR photons to produce the Xe* 6s[ $I1 level which subsequently fluoresces at 147.0 nm. As listed in Table II, 6p[&, [ $12, and [ $1, levels are responsible for the near-IR emission ( 12). It should be noted that the ratio of the fluorescence intensity at u’ = m to m + 1 is much smaller in Figs. 3 (b) and (c) than in (a) f suggesting that the radiative process and the predissociation are competing at 2)’ = m. The repulsive curve might cross the upper bound state between 2)’= m and m + 1. The alternative explanation is that the repulsive curve associates with the Xe* ( 5p55 d) levels which undergo radiative cascading (Xe 5 d --, 6p * 6s). This mechanism, however, would be of minor importance because radiative lifetimes of the Xe* ( 5p55 d) manifold are long (several ps) whereas there is no such slow component in the risetime of the VUV fluorescence. Our experiment reveals that at least three molecular systems, namely system III, system IV, and the new band system, are located near the Xe* 5 d[i]i level. The
317
SPECTROSCOPY OF Xe2 TABLE II
Summary of Term Energies, Branching Ratios, and Emission Wavelengths for Xe* ( 5p56p) States
notation
energy
final
state
branching
(cm-l,
wavelength
ratio
tnm>
6~[1/210
80119.5
6s[3/211
0.998
828.0
6p[3/212
79213.0
6s[3/212
0.700
823.2
6s[3/211
0.300
895.2
6s[3/212
0.084
840.9
6s[3/21,
0.915
916.3
6p[3/211
78956.5
combination of the two atomic terms 5p6 ’ SOand 5 d[ $1r gives rise to two ungerade states, OU+and 1U. Castex and Damany ( 1, 2) tentatively assigned systems III and IV to OU+ and lu, respectively. If one assumes that system IV associates to the 5 d[$, level, a potential hump of -200 cm-’ above the asymptote must be taken into consideration. All vibrational levels involved in the new molecular system are located below the 5d[i], dissociation limit, and consequently it seems more reasonable to ascribe the present system to one of the upper states associated with 5 d[ 21, level. The ab initio calculation might aid in establishing definitive assignments of these excited states.
According to Castex (3), system X is composed of two broad bands about 280 cm-’ apart (left column of Table III). In the present study, poor S/N ratio hampered
TABLE III The System X cas texa
This
transition wavenumber
V’ (cm-l)
work
transition wavenumber
(cm-‘)
82719.2 ‘)
82720.3b
82442.3
0
82445.8
82435.5
1
02457.7
82708.2
A
=
11.9
a Ref. (3). h This peak has a broad ( -5 cm-‘) feature superposed on a continuum.
cm-’
318
TSUKIYAMA AND KASUYA
a)
SYSTEM
v’=O
I
1 I
X
I
h
b)
82!4 00
82450
82500
cm-’ FIG. 4. Ruorescence excitation specrum of the system X: (a) 115 5 k,&. g 190 nm, (b) 800 G X0,. g 900 nm.
the detailed analysis of the band on the high-frequency side; the fluorescence intensity is one order of magnitude weaker than the new band system described in the previous section. The peak, superposed on a continuum, has a broad ( -5 cm-‘) feature. The band system on the low-frequency side shows a resolved vibrational structure as illustrated in Fig. 4. Both VUV (X 3 142 nm) and near-IR (800 S X S 900 nm) fluorescence were observed with the excitation of this band, suggesting that the upper state is predissociative. Transition wavenumbers are tabulated in Table III along with their vibrational numberings. A weak peak at 82 437.1 cm-’ is ascribed to a hot band (u’ = 1 + v” = 1) because the estimated vibrational wavenumber ( - 19.4 cm-’ ) of the ground state is in good agreement with the literature value of 19.9 cm-’ (4). The present system must relate to the Xe* 5 d[$ level since this level is the only one in the neighborhood The dissociation energy Do of the upper state is then calculated to be 170.1 cm-’ which is somewhat shallower than that of the ground state ( 185.2 cm-’ ) (4). The following values are derived: wk = 12.33 cm-‘, w,xk = 0.22 cm-‘, and De = 176.2 cm-‘. The Morse estimate of D, ( = wz/4o,x, + 173 cm-‘) is fairly consistent with the above value.
319
SPECTROSCOPY OF Xez
a)
v’
q
0 r
1
2
3
I
I
, SYSTEM
XI
b)
, 000
I 85100
I
1
#
85200 cm-'
FIG. 5. (a) Fluorescence excitation specrum of the system XI. Intensity of the VUV coherent radiation is almost constant over the entire region. (b) Calculated Franck-Condon factors: r: = 4.52 A.
Calculation of the Franck-Condon factors’ was carried out to evaluate the internuclear distance (rb) of the upper state. Morse potential curves were assumed for both ground and excited states. The molecular parameters for the ground state were fixed to the following values: w[ = 2 1.07 cm-‘, w,x: = 0.63 cm-l, and r Z = 4.362 A. The rotational constant B: of the excited state was varied until a reasonable agreement between observed and calculated intensities was achieved. For the upper state of system X, however, it was not possible to determine whether rk is greater or smaller than the ’ We have used the version from Dr. Jon T. Hougen of the National Institute of Standards and Technology for the RKR and Franck-Condon factor program which was originally written by Zare. Schmeltekopf. Harrop, and Albritton for use in their article in J. Mol. Spectrosc. 46, 37-66 ( 1973).
320
TSUKIYAMA
AND KASUYA
TABLE IV The System XI V’
V”
0
1
85047.2
0
0
85066.8
1
0
85091.0
2
0
85114.6
3
0
85137.7
transition
(cm-‘)
wavenumber
A
=
24.2
cm-l
23.6
23.1
ground state value, because in both cases the intensity of 2)’= 2 was calculated to be much smaller than that of 2)’= 1. To the best fit, r’, is 0.09 - 0.10 A apart from the ground state. The system IX around 82 000 cm-’ identified by absorption spectroscopy (4) could not be detected under our experimental conditions. This might reflect the magnitude of the absorption strength: the pressure of system IX was -40 Torr, whereas that of system X was only 5 Torr (3). Ill-C.The System XI
The system XI was described as an intense molecular system appearing at a pressure of3 - 6 Torr ( 3)) and was observed with a favorable S/N ratio as reproduced in Fig. 5 (a). Transition wavenumbers are tabulated in Table IV along with their vibrational numberings. As the progressions have the greatest intensity for the lowest u’, the most intense peak was assigned 21’= 0. The following spectroscopic constants are derived by the least-square method: ok = 24.8 cm-’ and w:x: = 0.28 cm-‘. Two nearby atomic levels, namely, 7s[$ (E = 85 189.31 cm-‘) and 7s[& (E = 85 440.53 cm-‘), could be proposed as the dissociation limit. The Do evaluated for these levels are 307.7 and 558.9 cm-i, respectively. The Morse estimate of Do (- 560 cm-’ ) favors the latter assignment. This system fluoresces only in the VUV region below 122 nm, indicating that the bound-bound transition between the upper and the ground state is dominant. We found that the intensity distributions were fairly consistent with FCF calculations with r: = 4.52 A as shown in Fig. 5(b). An attempt to fit with smaller internuclear distance than the ground state yielded less satisfactory results; the intensities of IJ’ = 2 and 3 were calculated to be about one and two orders of magnitude different from that of 2)’ = 1. In addition, (2)’ = 1 and 2) + v” = 1 transitions were estimated to have larger FCFs than the ~1’= 0 + 2)”= 1 transition. No significant difference was found by the adoption of Rydberg-Klein-Rees (RKR) potentials. In conclusion, laser-induced fluorescence study of jet-cooled Xez with spectrally strong, monochromatic VUV coherent radiation resulted in ( 1) the first observation of the molecular system in the vicinity of the 119.2~nm Xe atomic line and (2) the first accurate determination of the spectroscopic constants of systems X and XI. The symmetry of the upper states, however, could not be determined from the present
SPECTROSCOPY OF Xe2
321
study. The calculation of the potential curves correlating to Xe* ( 5p56p, 5p55 d and 5~~7s) is highly desirable for the better characterization of the excited states. IV. ACKNOWLEDGEMENT This work was supportedin part by the Kurata Foundation. The authors thank Dr. T. Suzuki for the loan of the Fabry-Perot etalon system and Dr. T. Ishiwata and Dr. T. Munakata for their instructions concerning Franck-Condon factor calculations. RECEIVED:
July 30, 199 1 REFERENCES
1. M. C. CASTEXAND N. DAMANY, Chem. Phys. Lett. 24,437-440 ( 1974). 2. M. C. CASTEXAND N. DAMANY, Chem. Phys. Lett. 13, 158-161 (1972). 3. M. C. CASTEX,Chem. Phys. 5,448-455 (1974). 4. D. E. FREEMAN,K. YOSHINO,AND Y. TANAKA, J. Chem. Phys. 61,4880-4889 ( 1974). 5. R. H. LIPSON,P. E. LAROCQUE,AND B. P. STOICHEFF, Opt.Lett. 9,402-404 (1984). 6. R. H. LIPSON,P. E. LAROCQUE,AND B. P. STOICHEFF, .I. Chem. Phys. 82,4470-4478 ( 1985). 7. K. TSUKIYAMA,M. TSUKAKOSHI,AND T. KASUYA, Chem. Phys. 127,393-397 ( 1988). 8. K. D. BONIN AND T. J. MCILRATH, J. Opt.Sm. Am. B 2, 527-534 ( 1988). 9. G. HILBER,A. LAGO, AND R. WALLENSTEIN,J. Opt. Sot. Am. B 4, 1753-1764 ( 1987). 10. W. L. GLAB AND J. P. HESSLER, Appi. Opt. 26, 3181-3182 (1987). 11. T. SUZUKIAND T. KASUYA, Phys. Rev. A 36,2129-2133 (1987). 12. N. BOWERING,M. R. BRUCE,AND J. W. KETO, J. Chem. Phys. 84,709-714 (1986).