4582
J. Phys. Chem. 1993,97, 4582-4588
Resonance Structure in the Vacuum-Ultraviolet Fluorescence Excitation and Multiphoton Ionization Spectra of IBr Andrew J. Yencha’ Department of Chemistry and Department of Physics, State University of New York at Albany, Albany, New York 12222
Trevor Ridley, Robert Maier, Robert V. Flood, Kenneth P. Lawley, and Robert J. Donovan Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, U.K.
Andrew Hopkirk SERC Daresbury Laboratory. Daresbury, Warrington, WA4 4AD. U.K. Received: December 1, 1992; In Final Form: February I O , 1993
Vacuum-ultraviolet fluorescenceexcitation and resonance enhanced multiphoton ionization (REMPI) spectra of IBr, cooled in a free-jet expansion, are reported. Resonance structure, observed in the region X > 179 nm, is greatly reduced on cooling and is ascribed to heterogeneous coupling between the E(O+) ion-pair state and the a6 Rydberg state. Observation of a new Rydberg series b6’[211,p]c6s(o+) X(O+) confirms the origin of thedip-resonancestructurein the fluorescenceexcitation spectrum at wavelengths 175 nm relative to the region 170 nm in comparison with UV/vis signal detection. In our previous work on IBr3 and more recently on we proposed that the E(O+) ion-pair state of these molecules is both homogeneously and heterogeneously predissociated. The homogeneous perturbation in IBr sets in at X d 179 nm, while the heterogeneous perturbation is observed at longer wavelengths. Both types of perturbation are manifest as a series of pronounced dips in the fluorescence excitation spectrum (as seen in the lower trace of Figure 1). The present results show very clearly the different nature of the two perturbations. Below 179 nm the dips sharpen on cooling, while at longer wavelengths the dips essentially disappear. The reason for the sharpening of the dips which arise from the homogeneous perturbation is readily understood: at room
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Yencha et al.
4584 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
v1 .e CI
C ¶
--Ee 2
.-
- 1
-E
.I.-x \
x
Y
9”
164
I
C W
Y
C
I
174
170
168
H
m
I72
166
I
176 178 Wavelength / nm
180
182
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Figure 2. Fluorescence excitation spectrum of jet-cooled IBr recorded with a resolution AA = 0.02 nm. The peaks labeled CY and CY’are the (265,O)and (264,0)179Br/(266,0)181Br excitation bands, respectively, in the E(O+) X(O+) system. The identification of these bands is 179Br/(267,0)InlBr based on the assignments given by Lipson and H o ~ The . ~ corresponding hot-band peaks for u” = 1 are identified as @ and j3’, respectively. The arrows in the lower panel mark the location of dip resonances discussed in the text.
temperaturethere is extensive overlapping of rotational structure from adjacent vibrational levels, and thus the dips are to some extent filled in by levels which are not predissociated. The reason for the disappearance of the dips due to heterogeneous perturbation is perhaps less immediately obvious. The key point here is that the perturbation is proportional to J, with higher J levels having a shorter predissociation lifetime and thus lower fluorescence quantum yield. As the effective rotational temperature of the molecules is lowered, the higher J levels will be depopulated, thereby reducing the effect of predissociation and producing a concomitantincreasein the fluorescencequantum yield. The present data therefore greatly strengthen our previous proposal that the dips in the fluorescence excitation spectrum of IBr, above h = 179 nm, are due to heterogeneous perturbation by a Rydberg state which is itself predissociated by a repulsive valence state. We identify the Rydberg state here as the a6[2113p]c6s(Q=l)state, as we did previously in ICl? although a more positive confirmation of the Q value (i.e., Q = 1 or 2) requires a higher-resolution study. This is currently being undertaken for both IBr and IC1. Our high-resolution fluorescence excitation spectrum of jetcooled IBr between 162.8 and 181.6 nm is presented in Figure 2. Thevariation in peak intensities over the entire spectral region shown is reliable as the normalized output from the synchrotron, together with the monochromator, is essentially constant over this wavelength range. As mentioned above, from spectral band contour simulations we would estimate a rotational temperature of 15 K. The higher-resolution jet-cooled spectrum shown in Figure 2 contains all the essential features of thelower-resolution, jet-cooled spectrum shown in Figure 1 (upper trace) except that in the former many more resolved ro-vibrational bands can be observed. The three dip resonances marked by the arrows in Figure 2 will be discussed more fully below. We have analyzed our high-resolution spectrum based on the Dunham expansion relationship established by Lipson and Hoy.4 An excellent fit was obtained over the entire spectral region if we assume significant contributions from v” = 0 and 1 but not from v” = 2. This is substantiated by the clear, unobstructed observance of a set of two hot-band structures for v” = 1 for the overlapping isotopomers labeled ,8 and in Figure 2 (identified as (265,1)179Br/(267,1)181Br and (264,1)179Br/(266,1)181Br,
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respectively). The corresponding set of bands for v” = 0 are labeled cr and a’in Figure 2. There is no indication of bands for v” = 2. Our fit is demonstrated in detail in Figure 3, showing the spectral region of specific interest in this paper, namely, h > 175 nm. As can be seen in Figure 3, there is a clear contribution from v”= 0 in this wavelength range, especiallynoticeablebetween 176 and 179 nm. At wavelengths >179 nm up to 181 mm, the bands for the two isotopomers for v”= 0 and 1 begin to coincide, and the spectrum of the four components increasingly appear to become a single progression. At our level of resolution (0.02 nm), it is not possible to ascertain the degree to which v” = 0 contributes to the spectral features in this wavelength region. The dashed lines in thevibrational assignmentsin Figure 3 indicate the positions of missing (or substantially reduced in intensity) bands in the fluorescenceexcitation spectrum that constitute the dip resonances mentioned above and discussed below. Three REMPI spectra of IBr are shown in Figure 4, in the two-photon excitation region 58 000-55 000 cm-I (172.4-181.8 nm). They were obtained by employing a room temperature static cell (top trace) and a free-jet expansion with nozzle backing pressures of Ar (plus IBr) of 0.6 and 1.7 atm (middle and bottom traces, respectively). Two Rydberg states with the [2111/2]c6s configuration are identified. The stronger of the two series is the b6(8=1) Rydberg transitionobservedbya (2+ 1) REMPIprocess. The weaker series, most clearly seen in the jet-cooled spectra shown in Figure 4 (middle and bottom trace), is identified for the first time as the bs’(Q=O+) Rydberg transition by analogy with a similar weak system in IC1 observed using both synchrotron9 and laserlo radiation. The nature of transitions from the ground state of IC1 and IBr to the bg and b6/ Rydberg state and the interaction of these Rydberg states with the E(O+) ion-pair state are discussed in the Appendix. In the present work, the b6’ state is observed either via a (2 + 2) REMPI process or by means of autoionization of a Rydberg state with a X *II1/2 corell formed in a (2 + 1) REMP excitation process, because at the wavelength of the observed (0,O)band, three photons do not provide sufficient energy to directly ionize the molecule (see arrow in Figure 4). The two-photon vacuum wavenumbers for the bandhead positions of these two systems are given in Table I. The values of the b6 seriesagree well with those previously obtained from an absorption study using synchrotron radiation.3 The higher-frequency struc-
Resonance Structure in Spectra of IBr
175
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4585
177
176
178
180
179
181
Wavelength I nm Figure 3. Detailed assignment of the fluorescence excitation spectrum of jet-cooled IBr in the long wavelength region: the assignments are based on the work of Lipson and H o ~ Vertical .~ dashed lines in the assigned progressions indicate missing or very weak band structure due to dip resonances as described in the text.
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x 8.5
A
static cell jet
0.6 atm Ar
A 58000
I
1
I
I
57500
57000
56500
56000
Two-photon energy /
jet
LA
\L
1.7 atm Ar I
55500
55000
ern-'
Fipre4. REMPIspectraofIBrinthe two-photonexcitationenergyregion58000-55 OOOcm-' (172.4-181.8nm) usingastaticcellatroomtemperature (top trace) and a free-jet expansion system with nozzle backing pressures of Ar (plus IBr) of 0.6 and 1.7 atm (middle and bot1) and b6'[21&/2],6s(0+)Rydberg systems are identified. The arrow tom traces, respectively). The observed bandhead positions of the b6[Zn~/2]c6S( in the figure at 55 867 cm-I marks the two-photon energy equivalent to the adiabatic ionization potential of IBr (83 801 cm-I)I2forming the X(*I11/2) state of IBr+ in a (2 + 1) REMPI process.
ture, most clearly seen in the 1.7-atm Ar, jet-cooled IBr REMPI spectrum (see Figure 4, bottom trace), running under the Rydberg systems between -56 700 and 58 000 cm-I, is due to two-photon excitation of the E(O+) ion-pair state." We note that the vibrational cooling achieved with the smaller jet size and higher backing pressures employed for the REMPI experiments is very much better than that achieved in the synchrotron work. In Figure 5, we compare the fluorescence excitation spectrum of jet-cooled IBr with the REMPI spectrumof room temperature IBr, in the wavelength range 175-181 nm. Also shown are the observed and predicted positions of several vibrational levels of the b6' system. (Note that the stronger b6 system has not been
marked to avoid congestion.) The predicted values were derived from o values observed in the b6 system. As can be seen, there is a closecorrespondencebetween the positionsof several members of the b6' progression, but not the b6 progression, with the dip resonances in the fluorescence excitation spectrum, as indicated by the vertical dashed lines in Figure 5. The vibrational levels of the ion-pair state that are involved are indicated in Figure 3. The least broadened dip resonance at 178.1 nm corresponds to the loss of two vibrational bands within the ion-pair state progressions(one from each isotopomer) of ut'= 1 hot-band origin. The Rydberg level with an equivalent transition energy would be u' = 2 of the bgl Rydberg system: this is clearly seen from the
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4586
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174.55
numerous degeneracies between the vibrational levels of the two states, thus causing extensive fluorescence loss from the ion-pair state. Finally, we attempt to determine where the E(O+) ion-pair state potential “crosses” the b6’(O+) Rydberg state potential. On the basis of the spectroscopic evidence provided in Figure 5, it appears that the first dip resonance a t 178.1 mm is associated with u”= 2 of the b i Rydberg state (cf. the (2,l) band). There is no indication of dip resonances associated with u’ = 1 and 0 of the bgl Rydberg state. Thus, we propose that the interaction between the bg’ Rydberg state and the E(O+)ion-pair state begins close to the u’ = 2 level and probably somewhere between the u’ = 1 and 2 levels of the bgl Rydberg state, thus placing the ‘‘crossing’’ in the E(O+) ion-pair state between u’= 223 and 229 in 179Brand between u’ = 225 and 231 in lalBr, as illustrated in Figure 6.
173.65
Conclusions
TABLE I: Measured Two-Photon Bandhead Positions and Separations for the b6(l) X(O+) and b6‘(O+) X(O+) Systems from REMPI of IBr b6
v‘, v ‘ ’ ~ ;b/cm-l 0,2
A;/cm-I
55 846
b6’
X/nm 3fcm-I 179.06 55272
266 0,l
56 112
178.21
56 379
55 541
177.37
56684
179.19 306
176.41
56985
180.05
55806 56 112
301 2.0
X/nm 180.92
265
305 1,0
A;/cm-l 269
267
0,O
178.21 305
175.48
56417
177.25
304
3,O
Yencha et al.
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
57 289
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297 4,O
57 586
Main component assignment. Estimated error in peak position A; = f 2 cm-I. 0
position of the (2,l) band in the REMPI spectrum. The same two vibrational levels in the ion-pair state must also be involved in the loss of fluorescence from transitions involving the u” = 0 level of the ground electronic state. Again, this can be seen from the location of the (2,O) band of the bgl Rydberg series, which coincides with the dip resonance located at 177.2 nm. In this wavelength region (Le., between -177.0 and 177.3 nm), four ion-pair hot bands are found to be missing or substantially reduced in intensity. These are associated with u’ = 2 and 3 of the bgl Rydberg state. At still shorter wavelengths (e.g., between 176 and 176.5 nm), the corresponding four ion-pair transitions close to u’ = 3 and 4 of the bgl state are also found to be missing or substantially reduced in intensity. At wavelengths below 176 nm, the presence of higher vibrational levels of the bgl Rydberg state (i.e., v ’ 2 4 which periodically coincide in energy with ionpair levels, cause the loss of fluorescence from higher lying ionpair levels. The mechanism for the loss of ion-pair fluorescence in the wavelength region
