3430
J. Phys. Chem. 1995, 99, 3430-3437
Electronic Spectroscopy and Excited State Dynamics of the KrAIH(XIZC',AIII)van der Waals Complex Eunsook Hwang and Paul J. Dagdigian" Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218-2685 Received: September 15, 1994; In Final Form: December I , 1994@
The laser fluorescence excitation spectrum of the KrAlH van der Waals complex, in the vicinity of the A'IIXIZ+ (0,O) band of diatomic AlH, is presented. This species was prepared in a pulsed free jet by 193 nm multiphoton dissociation of trimethylaluminum seeded in inert gas mixtures. The KrAlH bands were found to be both rotationally resolved and diffuse. Rotational analyses were carried out for most of the sharp bands. In contrast to the analogous ArAlH complex, the only observed bands from the ground KrAlH(X) bend-stretch level involved perpendicular [P= 1 P" = 01 transitions. In addition, six hot bands were observed and allowed determination of the spacing between the lowest P" = 0 and 1 levels. It was possible to assign the bend and van der Waals stretch vibrational quantum numbers for the observed KrAlH(A) bendstretch levels by comparison with previous experimental studies in our laboratory on the isovalent ArBH and ArAlH complexes and concurrent theoretical calculations by Alexander and co-workers on these systems.
-
1. Introduction
The nonbonding interaction of open-shell diatomic free radicals with inert gas atom partners has received considerable attention because these interactions govern the gas-phase collision dynamics of these species. Knowledge of such interactions is also helpful in interpreting the structure and dynamics of cryogenic media containing such diatoms. Considerable experimentally derived information on these interactions, principally the attractive portions of the potential energy surfaces (PES's), for both ground and electronically excited states has been inferred from spectroscopic studies of van der Waals complexes of these species.'s2 Most of the systems studied have involved complexes of the first-row hydrides and their corresponding deuteride isotopomers, and the most commonly employed rare gas partner has been argon. For some of these systems, ab initio calculations of the PES's have been performed, and these have been used as templates for the estimation of PES's which can reproduce the experimental spectroscopic observations. Complexes of Ar with the first-row hydrides OH, NH, CH, and BH in open-shell electronic states have now been experimentally observed. The first and most intensively studied complex is ArOH(X211,A28+),for which both ultraviolet laser fluorescence excitation spectra, at several levels of spectral resol~tion,~-* and microwave absorption spectra9 have been obtained. In addition, the excited state predissociation dynamics has been probed by line width measurements in the excitation spectrum,5.'0-12by spectrally resolved emission studies," and, recently, by stimulated emission detection of the electronically excited diatom fragments. l 3 3 l 4 Bend-stretch energies in the ground electronic state have been measured in stimulated emission pumping e ~ p e r i m e n t s . ~Several ~ J ~ g r o ~ p s ' ~have -~~ used these experimental data to derive phenomenological PES's, based on the ab initio PES's of Degli Esposti and for both the ground X 2 n and excited A2Z+ electronic states. For a diatom in an orbitally degenerate electronic state, e.g., the OH(X2n) state, two PES's, of A' and A" symmetry, are required to describe completely the atom-diatom interaction.
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracrs, February 15, 1995.
0022-365419512099-3430$09.00/0
In the case of ArOH(X211),the difference between these PES's is small and can be treated as a perturbati~n.~*>*~ By contrast, this difference potential, which has important consequences in the A doublet propensities in state-to-state rotationally inelastic cross section^,*^^^^ is significant compared to the spacing between asymptotic diatom rotor energy levels for ArCH(X211), ArBH(AIII), and ArNH(c'II). The laser fluorescence excitation spectrum of ArCH has been observed near the B2Z--X211 (0,O) and (1,O) bands of free CH.26 A significant parity splitting in the ArCH(X2n) electronic state was inferred from the rotational analysis of the bands. This splitting is a consequence of the difference p ~ t e n t i a l , and ~~.~~ the magnitude of the observed splitting has been reproduced in calculations based on ab initio ArCH(X211) PES'S.^* The laser fluorescence excitation spectrum of ArNH has also been observed in the spectral region near the c'lI-a'A (0,O) band of free NH.29 The rotational structure in the spectrum was found to be quite complicated, as a result of the small anisotropy of the ArNH(alA) PES'S,^^ and has been interpreted3' with the help of recently calculated ab initio ArNH(c'lI) PES'S.^* The pattern of ArNH(clII) bend-stretch levels predicted from these PES's is complicated, in part because of the large difference potential for this system. We have observed in our laboratory the laser fluorescence excitation spectrum of the ArBH complex near the AIII-XIZ+ (0,O) band of free BH.33 In contrast to the electronic spectrum of ArNH,29131the rotational structure of the sharp bands was found to be simple and as expected for a singlet spin multiplicity species, except for transitions to ArBH(A) bend-stretch levels near the dissociation limit. In a collaborative effort, Alexander et al.34 calculated ab initio ArBH(X,A) PES's and used these to predict the bend-stretch energies of ArBH in its ground and first singlet excited electronic states. These calculations were invaluable and provided the basis of a vibrational assignment of all the observed bound ArBH(A) bend-stretch levels except those just below the dissociation limit. In this analysis, an adiabatic bender model, in which the bending and intermolecular stretching vibrational motions are ~ e p a r a t e d ,was ~ ~ ,found ~ ~ to predict quite well the exact bend-stretch energies. The pattern of ArBH(A) bend-stretch energies is complicated and not
0 1995 American Chemical Society
Spectroscopy of KrAlH(X'Z+,A'II) Complex related in a simple way to the asymptotic diatom rotor energy spacings, in part, because of the large difference potential.34 There have been two examples of spectroscopic studies of van der Waals complexes of second-row hydrides with argon. have observed the laser fluorescence excitation Yang et spectrum of the ArSH complex, which is isovalent with ArOH, in the vicinity of the A2F-X211 (0,O) band of free SH. In the free diatom, the excited state is strongly predissociative; however, the rate of electronic predissociation is found to be significantly reduced by complexation with argon.36 In our laboratory, we have recently observed the spectrum of the ArAlH complex,37the second-row analog of ArBH. As with ArBH, these experiments on ArAlH were carried out collaboratively with Alexander's group, who computed ab initio ArAlH(X,A) PES'S and bend-stretch energies of the complex in both electronic states3* Because of the larger binding energy and the more closely spaced asymptotic diatom rotor levels, there are many more bound bend-stretch levels in ArAlH than for ArBH. With these calculations, it was possible to assign essentially all of the observed3' ArAlH(A) bend-stretch levels. In contrast to ArSH, the decay lifetimes of various bend-stretch levels of ArAlH(A) were found to be smaller than for free AlH.37 These observations imply that complexation with Ar causes electronic quenching. In the present paper, we extend our study of van der Waals complexes of group 13 hydrides to the KrAlH complex. We expect the binding energy of KrAlH to be larger than that for ArAlH. Measurement of decay lifetimes of excited KrAlH(A) bend-stretch levels will allow an assessment of the importance of electronic quenching in this species. As with ArAlH,37 several hot bands have been observed. This provides information on the vibrational energy spacing in the KrAlH(X) ground electronic state.
2. Experimental Section The KrAlH van der Waals complex was formed in a free jet expansion and detected through laser fluorescence excitation. The apparatus employed for these experiments has been described in detail p r e v i ~ u s l y .Mixtures ~ ~ ~ ~ ~ of the photolytic precursor for AlH, i.e., trimethy1aluminum4 (room-temperature vapor of 8.4 To&'), diluted in helium-argon-krypton mixtures (pressure 7-9 atm) were supersonically expanded into vacuum through a pulsed solenoid valve, whose nozzle orifice was 0.2 mm diameter. The krypton partial pressure in the backing gas was controlled with a flow meter. Dissociation of the precursor was carried out with an ArF excimer laser (Lambda Physik EMGlOlMSC) whose output was attenuated and focused at the tip of the nozzle orifice. The KrAlH complexes and free AlH molecules were detected 1 cm downstream of the nozzle by fluorescence excitation with the fundamental output of a dye laser (Lambda Physik LPD3002E) in the wavelength region of the AlH A1ll-XIZ+ (0,O) band near 426 nm. This laser was employed in both a low- and high-resolution mode (line widths 0.3 and 0.04 cm-', respectively). Typical dye laser pulse energies were 20-300 pJ in a 3 mm diameter beam. The dye laser was fired at a variable delay after the photolysis laser to allow for the molecular transit time from the photolysis to the detection zones. The fluorescence excited by the dye laser was imaged with a telescope, spectrally filtered with a l/4 m monochromator (Jarrell-Ash), and detected with a gated photomultiplier (EM1 9813QB), whose output was directed to a gated integrator (Stanford Research Systems SRS 250) and then to a chart recorder andor laboratory computer. A 125 MHz digital oscilloscope (Tektronix 2430A) was employed for the measure-
J. Phys. Chem., Vol. 99, No. 11, 1995 3431 n I
I
I
l
l
I
12
I
I
~
I
I
I
1
I
1
i '
'
"
1
'
'
'
11
'
i
'
"
R
I I I I l l r h
16
1
1
2 O
7
'
1
i i
6 1 0
'
'
'
~
l
23300
23350 23400 23450 23500 laser wavenumber (cm- '1 Figure 1. Survey laser fluorescence excitation scan of the spectral region around the origin of the AlH A'lI-X'Z' (0,O)band. The R, Q, and P branches of this band are marked. Also shown in this spectrum are bands assigned as transitions (denoted as A0 through E) in the KrAlH van der Waals complex. The dye laser pulse energy was approximately 50 pJ during this scan. ment of excited state decay lifetimes, as described in detail previou~ly.~~,~~,~~ A portion of the output of the dye laser was directed through a solid fused-silica etalon in order to monitor wavenumber differences as the laser was scanned. Wavelength calibration was carried out using the lines of the diatomic A M AIII-XIZ+ (0,O) band, whose line positions are given by Herzberp [Table 251 as obtained in a study by Bengtsson-Knave and for which derived constants are also reported by Zeeman and Ritter.45 Transition wavenumber differences from adjacent diatomic AlH lines could be determined to f 0 . 2 cm-' when the dye laser was operating in the low-resolution mode and to f 0 . 0 4 cm-' in the high-resolution mode. Unfortunately, the accuracy of the A1H transition wavenumbers (employed here for absolute calibration) obtained from the l i t e r a t ~ r eis~ somewhat ,~~ suspect because of discrepancies between the line positions given in Herzberg' s b o o P and those calculated from the constants of Zeeman and Ritter.45 We estimate that our reported absolute ArAlH transition wavenumbers are accurate only to &OS cm-'.
3. Results 3.1. Observed KrAlH Bands. Figure 1 presents a lowresolution survey spectrum in the vicinity of the Q branch band head of the A1H A'II-X'Z+ (0,O) band. The vertical scale of the plot has been expanded in order to reveal the weaker features in the spectrum. The strongest features involve resolved rotational transitions in the electronic spectrum of the A1H diatom. As noted in our spectroscopic study of ArAlH,37the A1H rotational temperature was 6-7 K, as determined by the ratio of the R(0) and R(1) lines. However, there is a high-j non-Boltzmann tail to the rotational state distribution, and there are visible in the spectrum rotational transitions out of levels as high as j" = 18, beyond which the fluorescence quantum yield becomes negligible because of excited state rotational predissociation throggh a potential barrier.M We also can see a number of features (denoted A0 through E) other than A1H lines in the spectrum displayed in Figure 1. These are assigned as electronic transitions in the KrAlH complex for a number of reasons. These features are present only when there is a significant mole fraction of Kr in the highpressure beam backing gas mixture. The AlH chromophore
Hwang and Dagdigian
3432 J. Phys. Chem., Vol. 99, No. 11, 1995 TABLE 1: Transition Wavenumbers (in cm-') for the Observed KrAlH Bands sharp bands" diffuse bandsb band ID
V
A0
23 296.7 23 309.8 23 322.2 23 339.9 23 353.2 23 365.2 23 380.3 23 393.8 23 406.0 23 433.0 23 466.9 23 522.3 23 650.3 23 655.2 23 671.58
A A2 BO B B2
co
C c2
D
E F G H I
&
43.4d 40.6' 39.9
bandID
v
J K L M N 0 P
23 686.9 23 698.9 23 707.0 23 710.3 23 724.4 23 739.1 23 752.5 23 762.2 23 771.6 23 777.8 23 782.3
Q R
33.9 55.4 128.0 4.9 16.3
S T
AC
12.0 8.1 3.3 14.1 14.7 13.4 9.7 9.4 6.2 4.5
=Estimated uncertainties in the band head positions f 0 . 1 cm-'.
* Quoted wavenumbers refer to the maximum intensity of the band. Estimated uncertainties f 0 . 3 cm-'. Wavenumber differences between this band and the next one. Wavenumber difference between bands A and B. e Wavenumber difference between bands B and C. f Wavenumber difference between bands C and D. g A high-resolution scan of this band was not taken since this band is weak. Hence, this band could be diffuse. must be present in the molecular carrier since these transitions occur very close in wavenumber to the free A1H electronic transition, and the excited state decay lifetimes are comparable to that for electronically excited diatomic A1H. For all the bands for which we have taken high-resolution scans, it has been possible to resolve rotational structure within these features. The derived rotational constants are consistent with the KrAlH chemical formula and do not admit the possibility of more than one Kr atom in the complex. The measured transition wavenumbers of the JSrAlH band heads are reported in Table 1, where they are identified by letters for a compact notation. In Figure 1, we also see six weak bands, denoted AO, A2, BO, B2, CO, and C2, which have intensities of the order of 5-10% of the neighboring strong bands (A, B, and C). These are assigned below as hot band transitions. A similar set of four hot bands was also observed in the spectrum of ArAlH.37 The high-energy portion of the KrAlH spectrum is presented in Figure 2. The features labeled F through T are ascribed to the KrAlH complex. The intensities of these features vary uniformly with the intensities of the strong bands displayed in Figure 1 as the Kr mole fraction in the seed gas is varied. We have taken high-resolution scans for a number of the bands shown in Figure 2. We find that bands G and H both show resolved rotational lines whose widths are comparable to the laser bandwidth. By contrast, high-resolution scans of bands J, K, and N do not reveal any rotational structure. The transition wavenumbers of all the bands assigned to KrAlH, except the very broad features to the blue of band T, are reported in Table 1. Since band J and higher bands appear diffuse, we deduce that an upper bound for the convergence limit to the lowest rotational asymptote in the electronically excited state, namely, AlH(A'II,v'=Oj'=l) Kr, is 23686 cm-'. As we have previously observed for both ArBH and ArA1H,33,37 there appear in the spectrum of KrAlH several series of diffuse bands which converge to higher-energy limits, which should correspond to higher rotational asymptotes. One series includes bands N through T and terminates at a limit near 23 787 cm-'. There also clearly appears in Figure 2 a second, higher-energy series of broad bands which converge to a limit near 23 852 cm-'. If
+
23500
23600
23700
23800
laser wavenumber (cm- 1) Figure 2. Survey laser fluorescenceexcitation scan of the higher energy bands (denoted as F through T) assigned to the KrAlH van der Waals complex. The lines of the R branch of the A1H A'II-X'X+ (0,O) band are also marked, and the assigned convergence limits to the AlH(A'n,v'=Oj') + Kr rotational asymptotes are indicated. The dye laser pulse energy was approximately 50 pJ during this scan. the upper bound of 23 686 cm-' is close to the j' = 1 asymptote, then we predict the j' = 4 and 5 asymptotes to occur at 23 794 and 23 855 cm-', respectively. The above-described convergence limits are only slightly lower than these estimated asymptotic energies and may be reasonably assigned to these asymptotes. On this basis, we can thus revise downward our estimate of the energy of the lowest rotational asymptote, to 23 681 f 2 cm-'. The diffuse bands in the spectrum represent transitions to metastable, predissociating vibrational levels supported by bender curves correlating with AlH(j'> 1) Kr asymptotes. Predissociation occurs by coupling with repulsive bender curves correlating with lower rotational asymptotes. We also expect to observe such diffuse bands converging to the j' = 2 and 3 asymptotes. There are some weak bands in the region 23 690-23 740 cm-' in the spectrum shown in Figure 2, but it is not possible to organize these convincingly into series of bands converging on these limits. From the wavenumber difference between our estimate of the energy of the lowest AlH(A) Kr rotational asymptote and the diatomic A1H R(0) line [23 483.54 cm-' @I, we derive the ground state KrAIH(XIZ+) dissociation energy DO'' = 198 f 2 cm-'. The wavenumber difference between the assigned limit at 23 681 f 2 cm-' and the redmost band involving a transition from the ground KrAlH(X) vibrational level (band A) provides a lower bound to the excited state KrAlH(A) dissociation energy Dol. We thus obtain the excited state binding energy DO' 2 371 f 2 cm-'. 3.2. General Considerations in the Rotational Analysis. Our previous theoretical and experimental papers33,34,37,38 on the ArBH and ArAlH complexes have presented a detailed discussion of the expected rovibrational transitions in the electronic spectrum of a van der Waals complex of a diatom undergoing a 'II 'Z electronic transition. In these complexes, an adiabatic bender in which the bending and intermolecular stretching vibrational motions are separated, was extremely useful in predicting the energies of the lower bendstretch levels. We can ignore the diatom vibrational degree of freedom (v = 0 here in both the upper and lower electronic state). As the atom partner approaches the diatom, a rotational level j will split into several bender curves, with different projection: P (in the notation of Dubemet et a1.**) along the direction R which connects the atom with the diatom center of mass. Each of these bender curves, which define potential
+
+
-
Spectroscopy of KrAlH(X’P,A’ll) Complex
I
n
/
I
J. Phys. Chem., Vol. 99, No. 11, 1995 3433
’
li I
I
Figure 3. Schematic diagram of the various angular momenta of an atom-diatom van der Waals complex such as KrAlH. energy curves as a function of the atom-diatom separation R, supports van der Waals stretching vibrational levels (denoted with the quantum number vs). The total angular momentum of the complex, which is the sum of the diatomic rotational angular momentum j and the orbital angular momentum 1 (end-overend rotation of the @atomic complex), is denoted as J ; the projection of J along R is also P since I must be perpendicular to the triatomic plane. If Coriolis coupling is small compared to the separation between bend-stretch levels with values of P,then P will be a good quantum number. The various angular momenta of the complex are shown schematically in Figure 3. We expect the pattern of the bend-stretch levels of KrAlH to be similar to that for ArAlH. Thus, we expect the lowest KrAlH(X) bend-stretch level to have P“ = 0. As in the spectrum of ArAlH,37we have observed hot bands in the KrAlH spectrum. Likewise, we expect the first excited KrAlH(X) bend-stretch level to have P” = 1. The general selection rule for the projection quantum number P in an electronic transition in a complex involving an open-shell diatom is AP = 0, f1,22,34,38 The electronic degeneracy of the KrAIH(AIII) excited electronic state leads to a doubling of the number of bender curves correlating with a given AlH(A,j’) Kr asymptote. We have d e n ~ t e dthe ~ ~two . ~ P’ ~ = 0 bender curves correlating with the same asymptote as P = O+ and 0-, since these will have the rotational structure analogous to those of diatomic Hund’s (c) S2 = O+ and 0- states, respectively. That is, the total parity of rotational levels for P‘ = O+ and 0- is +(-l)J and -(-l)J, respectively. This further implies that parallel (AP= 0) electric dipole radiative transitions from the ground KrAlH(X) bendstretch level can occur only to P = O+ level^,^^,^^ in exact analogy to the diatomic selection rule O+ O+.* By contrast, perpendicular (AP = f l ) transitions can, in principle, occur to both the lower or upper P = 1 level correlating with a given asymptote. The absence or presence of a Q branch in the rotational structure characterizes a parallel or perpendicular transition, respectively. The most strongly bound bend-stretch levels in ArBH,33334 ArA1H,37*38 and, by inference, KrAlH are a set of levels with P = 0-, 1,2, .... Vibrational motion in the orbitally degenerate excited electronic state involves, in principle, two PES’s, of A‘ and A” symmetry. Alexander et al.34have considered which PES(’s) determine the nuclear vibrational motion in the various bender curves and concluded that the P = Of and 0- levels involve motion only on the A’ and A“ PES’s, respectively. No such rigorous general separation was found for the P > 0 levels. However, the A” PES for ArBH(A),34ArA1H(A),38and quite likely also KrAlH(A) has a much larger binding energy than the corresponding A‘ PES. Consequently, all the strongly bound bend-stretch levels, with P = 0-, 1, 2, ..., were found to involve nuclear motion essentially only on the A“ PES.34s38The spectroscopic selection rules discussed above allow electronic
+
-
AlH(X’C+) + Rg
RgAlH(X)
Figure 4. Schematic diagram (not to scale) of the expected bender curves of a RgAlH(X,A)van der Waals complex (Rg = rare gas). The correlations with the AIH Rg rotational asymptotes are given. Each bender curve can support several stretch levels (not specifically indicated in the figure). The AlH R(0) transition and the various allowed P P‘ transitions are marked. The assignments of the observed KrAlH bands, presented in section 3.3, are also given. In contrast to ArBH (ref 33) and ArAlH (ref 37), no P = O+ P‘ = 0 bands were found.
+
-
-
transitions from the ground P” = 0 level to the strongly bound set of levels only with P = 1. However, more weakly bound bend-stretch levels, which involve correlated motion on the two PES’s and have P = O+ and 1, are also accessible from the lowest P“ = 0 level. Hot band transitions from the low-lying P” = 1 level can, in principle, occur to the strongly bound P’ = 0-, 1, and 2 levels. Yang et calculated the band strengths for various transitions in the ArAlH complex. They found that the P = 1 P‘ = 1 transition has negligible strength, and only transitions to the lowest P = 0- and 2 bend-stretch levels should be observable, because of the well-defined electronic symmetry (A”) of these strongly bound excited bender curves. These hot band transitions should lie close to the corresponding transition from the ground P‘ = 0 level to the P = 1 level. These expectations, which are diagrammed schematically in Figure 4, were confirmed in the experimental ArAlH spectrum.37 Similar considerations should apply to KrAlH. In the rotational analysis of the resolved bands, described below, the wavenumbers of the rotational transitions could be adequately fit by the rigid-rotor formula for the end-over-end rotational energy. Thus, the transition wavenumbers were expressed as
-
v =T
+ B’(J’ + 1) - B” J”(J’’ 4-1)
(1)
where B’ and B” are the upper and lower state rotational constants, respectively, and T is the band origin. Small J-independent contributions to the rotational energy have hence been included in the band origin wavenumbers. Aluminum has only one stable naturally occurring isotope (27Al). However, there are five stable isotopes of Kr, with amu 80, 82, 83, 84, and 86; the last two are most abundant (56.9% and 17.4% of the total elemental abundance, respectively46).In several of the observed bands, we have been able to ascertain the isotope splitting between the 84386KrAlHisotopomers, as has been done previously in the spectra of other Kr-containing
Hwang and Dagdigian
3434 J. Phys. Chem., Vol. 99, No. 11, 1995
TABLE 2: Spectroscopic Constants (in cm-I) Derived from Rotational Analyses of KrAlH bands and Vibrational Assignments
(a) band A
bandID A B C
D E F l
'
"
23309
'
l
'
"
l
23310
'
"
'
l
'
23311
'
"
l
'
~
23312
"
l
23313
23314
(b) band C Q rnI\ ~
I
4
I
I
I
I
8
I
I
I
I
12
I
l " " I " " 1 " " I " " l " " l
23393
23394
23395 23396 23397 23398 laser wavenumber (cm- ') Figure 5. High-resolution laser fluorescence excitation scans of bands A [upper trace of panel a] and C [upper trace of panel b] of the ArAlH complex. The lower traces in both panels are simulated spectra using the constants given in Table 2 and assuming a perpendicular transition (P = 1 P" = 0) with a rotational temperature of 3 K. In panel b, the dashed line in the lower trace is a simulated spectrum with the 84KrAlH isotopomer only, while the solid trace also includes the 82.83386KrAlH isotopomers. The transition wavenumbers of lines in the R, Q, and P branches are marked. In the lower panel, the Q and P band heads of the 86KrA1Hisotopomer are indicated.
-
complexes, e.g., A l ~ and j The difference in the reduced masses of s4KrAlHand 86KrAlHis only 0.6%, and the corresponding difference in their rotational constants is within the uncertainty with which the rotational constants can be determined. Thus, the observed isotope splittings reflect primarily the shift of the band origins with isotopic substitution. Rotational analysis of resolved bands allows determination of not only rotational constants and excitation energies but also the excited state projection quantum number P', from the presence or absence of a Q branch. The determination of the other excited state bend-stretch quantum numbers, j' and v:, is somewhat more difficult. However, the Kr-AlH(A) interaction is expected to have a similar anisotropy to that for ArAlH(A) so that it should be possible to make these assignments for KrAlH by analogy with ArAlH. The next section describes our analysis of the KrAlH bands, determination of spectroscopic constants, and assignments of bend-stretch quantum numbers. 3.3. Rotational Analysis and Vibrational Assignments. High-resolution scans were taken of all the strong bands (A, B, C, D, and E) shown in Figure 1, as well as of band F. The first four bands were found to be blue degraded (hence, B' > B"), while the latter was red degraded (B' < B"). Moreover, all these bands showed a clear PQR branch structure. Thus, all these bands involve perpendicular P = 1 P' = 0 transitions. Figure 5 presents high-resolution scans over two of these bands (A and C). Rotational line assignments in each band were made with the help of ground state combination differences, initially using an estimated B" value, and by comparison of the experimental
-
A0 A2 BO B2 CO C2
P
j'"
vSla
originb
B"
C
B'
Transitions out of Ground KrAlH(X) Vibrational Level 1 11 0 23 310.2 0.0546(21)d 0.0653(17) 23 353.7 d 0.0632(17) 11 1 1 23 394.4 d 0.0612( 17) 11 2 1 23 433.1 d 0.0594( 17) 11 3 1 23 466.8 e 0.0568(10) 11 4 1 23 522.3 e 0.0516(12) 1, 0 1 02 02 02
1 21 1 21 1 21
0 0 1 1 2 2
Hot Bandsf 23296.7 23322.2 23339.9 23365.2 23 380.3 23406.0
"The quantities j' and v; are the asymptotic AM(A'n) diatom rotational quantum number for the adiabatic bender curve and the van der Waals stretching quantum number, respectively,assigned by analogy with ArAlH (refs 37 and 38). The subscripts 1 and u on the j' value for P 2 1 denote the lower and upper bender curves, respectively, of the same P and j'. Estimated uncertainties of wavenumber differences from A1H diatomic lines f O . l cm-l. Quoted uncertainties are one standard deviation in units of the last significant digit. dLower state constant obtained by a simultaneous fit of bands A, B, C, and D. e Fixed at the value obtained in the fit to bands A-D. fTransitions out of the lowest P' = 1level of KrAlH(X). High-resolution scans of these bands were not taken, and the reported transition wavenumbers refer to the band heads.
spectra with simulations. The assigned transition wavenumbers were fit to the rigid-rotor formula (eq 1) to determine the spectroscopic constants. The individual line positions were fit essentially to their measured accuracy, and the inclusion of centrifugal distortion constants as free parameters was not required, as expected from the small range of J levels observed. As a further check on our line assignments and the fits, the experimental spectra were compared with simulated spectra, as shown in Figure 5. It can be seen from Figure 5 that a KrAlH rotational temperature of 3 K reproduces the relative line intensities in the bands quite well. As noted previously for ArA1H,37 the temperature of the complex is somewhat lower than that required to describe the diatomic A1H j" = 0 to 1 rotational population ratio. We expect that all these bands involve transitions from the ground KrAlH(X) bend-stretch level. Consistent with this assumption, fits to individual bands yielded essentially the same lower state rotational constant B". The final fit for bands A-D involved a simultaneous fit of all four bands with the same lower state constant B". Subsequently, fits to bands E and F were carried out with the value of B" fixed to the value determined for bands A-D. The derived rotational constants and band origins for all the fits are presented in Table 2. For several of the bands, we have been able to observe Kr isotope splittings. Because the excited state dissociation energy is larger than for the ground state, we expect that, in general, the 86KrA1H bands will lie to the red of the corresponding 84KrAlHbands. Bands of the former isotopomer should be thus discernible in blue-degraded bands. For band A, we do not observe an isotope splitting and conclude that it is 10.05 cm-'. The Q and P branch head of bands B and C for @KrAlH are visible in the spectrum to the red of the stronger corresponding 84KrAlH bands, as is shown for the latter in Figure 5b. We deduce the isotope splitting for these bands to be 0.13 and 0.23 cm-', respectively. Unfortunately, the spectral region of the P branch of band D is complicated, and we have not been able to derive unambiguously the isotope splitting for this band. The
J, Phys. Chem., Vol. 99, No. 11, 1995 3435
Spectroscopy of KrAIH(XIC+,AIII) Complex spectra obtained for bands E and F were of poorer signal-tonoise ratio, and isotope splittings were not derived for these bands. From the previous calculations on ArBH34and ArAlH,38we expect the lowest-energy perpendicular transitions in the spectrum of KrAlH to involve excitations to the vs) = 0, 1, ... van der Waals stretch levels supported by the P = 1, j’ = 11 bender curve. (We are employing the notation of our previous papers33,34,37,38 and denoting the lower and upper P = 1 bender curves correlating with the j’ = 1 diatom rotor asymptote as j’ = 11 and l,, respectively.) From the large intensities of bands A-D and our excellent signal-to-noise ratio, it is clear that band A must involve excitation to the v i = 0 level supported by this bender curve. The spacing of the corresponding v l = 0 and 1 levels in ArAlH was found to equal 38.7 cm-1.37 If we assume that the harmonic force constant for the P‘ = 1,j’ = 11 bender curve is proportional to the binding energy, we predict the vs) = 0 and 1 spacing to be 44.4 cm-’. This is very close to the wavenumber difference between bands A and B (see Table 1). Thus, we assign band B as excitation of the P‘ = 1,j’ = 11,v: = 1 bend-stretch level. We also see from Table 2 that the upper state rotational constant B’ drops approximately linearly from bands A through E; likewise, there appears (see Table 1) to be a monotonic decrease in the spacings between these bands. Hence, we assign the upper levels of bands C , D, and E as the P‘ = 1, j’ = 11, :v = 2 , 3, 4 bend-stretch levels, respectively. These assignments for bands A-C are entirely consistent with the observed Kr isotope splittings. The one remaining band for which we were able to obtain a high-resolution scan and carry out a rotational analysis was band F. Because of the large spacing between bands E and F, the latter clearly does not involve a stretch level supported by the P‘ = 1,j’ = 11bender curve. The most reasonable assignment for the upper level of band F is the vs) = 0 stretch level supported by the next P‘ = 1 bender curve, namely, the j’ = 1, bender curve. This assignment is consistent with the assignments of upper levels of perpendicular bands in ArBH33 and ArA1H.37 As mentioned above, there are three pairs of bands, each of which lie close to bands A, B, and C. These bands cannot be assigned as excitations from the ground P” = 0 level. Rather, in analogy with similar bands in ArAlH,37we assign these bands as hot band transitions from the lowest JCrAlH(X) P” = 1 level. The spacings between each of the pairs (AO-A2, BO-B2, COC2) are approximately the same. Moreover, the spacings between these pairs are essentially the same as the spacings between bands A, B, and C. We thus assign the upper levels of these hot bands as the vs’ = 0, 1, and 2 stretch levels supported by the lowest P‘ = 0- and 2 bender curves, as given in detail in Table 2 and diagrammatically in Figure 4. Because of the low intensities of these bands, we did not take highresolution scans of the hot bands nor carry out rotational analyses. We may use these assigned hot bands to derive an experimental estimate for the energy of the P” = 1 level relative to the ground KrAlH(X) bend-stretch level. Unfortunately, the hot bands do not connect to excited state levels for which we have observed transitions from the ground P” = 0 level. In both ArBH34and ArAlH,38the energies of the lowest P‘ = 0-, 1, 2 , .,. levels are well fit by the formula for the K structure of a rigid symmetric top.49 Our observed spacings of bands AOA2, etc., can be parametrized by a rotational constant A’ = 6.4 cm-l. This estimate allows us to calculate the wavenumbers for the hypothetical transitions from the P“ = 1 level to the vs‘ = 0-2 stretch levels for P‘ = 1. From the spacings of these calculated transitions for bands A, B, and C, we estimate that
TABLE 3: Measured Decay Lifetimes for KrAlH(A) Bend-Stretch Levels decay lifetime (ns)O band ID P i’ V; A 1 11 0 W3) B 1 11 1 26i3) C 1 11 2 W3) D 1 11 3 36i4) Quoted uncertainties are one standard deviation in units of the last significant digit. the KrAlH(X) P” = 1 level lies 7.5 f 0.2 cm-’ above the ground bend -stretch level. 3.4. Decay Lifetimes. In our experimental study of the ArAlH electronic spectrum, we found that the decay lifetimes for various excited ArAlH(A) bend-stretch levels were significantly less than the radiative lifetime for free AlH.37 This increased excited state decay rate was ascribed to the presence of electronic quenching in the complex. While rotational predissociation of diatomic AIH(AIIT) is known to occur over a potential energy barrier for high j’ levels,44 dissociation of A1H is energetically closed for the excitation energies of the investigated levels of the complex. In the present study, we have measured excited state decay lifetimes for excitation of the stronger bands in the KrAlH spectrum. Table 3 presents the derived lifetimes for the P‘ = 1, j’ = 11, v i = 0-3 bend stretch levels. It can be seen that all these lifetimes are significantly smaller than the lifetime of 66 f 4 ns which has been reported by Baltayan and Nedelec50for free AlH(A,v’=O) in low rotational levels. This implies that, as in the ArAlH complex, the binding of AlH(A) to Kr allow an additional excited state decay mechanism in addition to purely radiative decay. The decay lifetime of the lowest-energy excited KrAlH(A) investigated, namely, the P‘ = 1, j’ = 11, vs’ = 0 bend-stretch level, is the same, to within the experimental uncertainty, as the decay lifetime (28 f 3 ns) which we previously measured37 for the corresponding level in ArAlH(A). This indicates that the electronic quenching rate is similar in the two complexes. We also see from Table 3 that the decay lifetimes increase slightly with increasing stretch quantum number vl, within the P’ = 1,j’ = 11bender curve. In our previous study of ArAlH,37 we observed that the decay lifetimes for less strongly bound bend-stretch levels were greater and approached that of free AlH(A). We attributed this to a decreased electronic quenching rate, as a result of larger average Ar-AlH separations. In similar fashion, the slight increase in decay lifetime vs increasing vs) for the KrAlH(A) P‘ = 1, j’ = 11 bender levels can be explained as arising from an increasing average Kr-A1H separation. Unfortunately, the intensities of KrAlH bands involving excitation of levels supported by other bend-stretch levels were not high enough to allow measurements of decay lifetimes of these levels.
4. Discussion Table 4 presents the derived dissociation energies for the ground and first singlet excited electronic state of the KrAlH complex. Also included in Table 4 are derived average KrA1H separations, defined as (R-2)-1/2 and obtained from the experimental rotational constants. Table 4 also compares the dissociation energies and average rare gas-diatom separations for the isovalent ArBH, ArAlH, and KrAlH complexes, all studied in our laboratory. It can be seen that for all these complexes the binding energy increases and the average separation decreases upon electronic excitation. The ground XIZ+ electronic state of a group 13 hydride has a closed-shell ...az electron configuration, while electronic
3436 J. Phys. Chem., Vol. 99, No. 11, 1995
Hwang and Dagdigian
TABLE 4: Comparison of Experimentally Determined ArBH, ArAlH, and KrAlH Binding Energies (in cm-') and Atom-Diatom Separations (in hi, complex Doll Do' R{ Ro' ArBHd
ArAlH' KrAlW
92 124 198
176 220 37 1
3.70 3.86 3.83
3.31 3.59 3.51
'Reported values are dissociation energies from the P' = 1, j' = 11, = 0 levels. Vibrationally averaged atom-diatom separation, . averaged separation, [(R-2)]-1/z, defined as [ ( R - z ) ~ ] - 1 / 2Vibrationally for the P = 1, j' = 11, v; = 0 bend-stretch level. dReference 33. e Reference 37. f This work. v:
excitation to the A ' n state involves promotion of one of the o electrons to a ~t orbital, yielding a ...onelectron configuration. Because of the orbital degeneracy of the excited state, two PES's arise in the interaction of a rare gas atom with the diatom in its A'II state. In ab initio calculations carried out in collaboration with our e x p e r i m e n t ~on ~ ~ArBH , ~ ~ and ArAlH, Alexander and c o - w o r k e r ~found ~ ~ ~that ~ ~the A" PES was much more attractive than the A' PES for both systems. In part, this is due to the decreased Pauli repulsion between the x electron and the rare gas atom when this electron occupies the component perpendicular to the triatomic plane. The most strongly bound bendstretch levels in ArBH(A) and ArAlH(A) are found to involve nuclear motion primarily on the more attractive A" PES.34,38 The decreased binding energy, and increased atom-diatom separation, in the ground electronic state of these complexes can likewise be explained by the increased Pauli repulsion of the rare gas atom with the diatom valence electrons in the u2 shell, as compared to that for the uJt(a") electron occupancy in the excited A" PES. We also see from Table 4 that the ground and excited state binding energies are significantly greater for KrAlH than for ArAlH. Similar increased nonbonding interactions have been observed in KrOH and AlKr, as compared to ArOH and AlAr, r e s p e ~ t i v e l y . ~Despite ~ , ~ ~ the expected larger van der Waals radius of Kr vs Ar, we see from Table 4 that the average KrAlH separation is slightly less than the Ar-A1H separation in both the ground and excited electronic states. By contrast, in the closed-shell rare gas-HFS1 and -HCl systems,52the binding energy and atom-diatom separation both increase monotonically as the rare gas partner goes from Ne to Xe. On the other hand, the equilibrium atom-diatom separation is slightly smaller for KrOH(A2Z+)2947than for ArOH(A2Z+).19 However, in comparison to other nonbonding interactions, the binding is quite strong in these latter two systems. We can interpret the slightly decreased atom-diatom separation in KrAlH vs ArAlH as an illustration that an increased attractive interaction can shift the onset of the repulsive interaction to shorter atom-diatom separations. The observed KrAlH bend-stretch energies provide considerable information on the anisotropies of the PES's. From our observation of hot bands, we have deduced that the first excited KrAlH(X) bend-stretch level, which has P" = 1, lies 7.5 & 0.2 cm-' above the ground (P" = 0) bend-stretch level. This spacing is only slightly greater than the corresponding spacing (7.2 f 0.1 cm-') deduced for the ArAlH(X) complex.37 In the theoretical investigation of this complex by Yang et U Z . ,the ~~ strongly bound ArAlH(X) bend-stretch levels have P" = j". The energies of these levels were found to be fit well by the formula for the K structure of a rigid symmetric top49 with a value for the A rotational constant close to the value calculated for the equilibrium geometry of this state, for which the angle 8 between the A1H diatom internuclear and Ar-AlH axes was found in the ab initio calculations38 to equal 72.6" (8 = 0"
corresponds to linear AlHAr geometry). The slightly larger value of the P' = 1 to P" = 0 spacing in KrAlH vs ArAlH implies a slightly smaller (by ~ 3 " equilibrium ) value for 8 in the former complex Yang et al.38 also found that the calculated energies of the lowest ArAlH(A) bender curves, with P' = OW, 1 , 2, ..., could also be fit well by the formula for the K structure of a rigid symmetric top. We find that the derived A rotational constants from analysis of the spectra of ArAlH and KrAlH are quite similar [6.5 and 6.4 cm-', respectively]. This implies that the equilibrium value for 8 is essentially the same for the A" PES's of both complexes. The strongest bands in the spectra of both ArAlH and KrAlH involve excitation to stretch levels supported by the P' = 1,j' = 11bender curve in the A state. This agrees qualitatively with calculated38band strengths for excitation of ArAlH(A) levels supported by the lower bender curves. In KrAlH, the intensities of bands involving excitation to levels supported by bender curves other than P' = 1, j' = 11 are weak, and we have been able to assign only one such band, involving excitation of the P' = 1, j' = l,, vs) = 0 bend-stretch level, in the spectrum. There are some very weak features in the KrAlH spectrum (see Figure 2) to the blue of the A1H R branch head. These could involve excitation of other bound KrAlH(A) bend-stretch levels, but their intensities were too small to warrant highresolution scans. The splitting between the P' = 1,j' = 11 and 1, bender curves is related to the difference between the A' and A" PES's in the excited electronic In ArBH,33the energy difference between the corresponding vs) = 0 levels supported by these two bender curves is 97.0 cm-', or 55% of the excited state dissociation energy given in Table 4. The energy difference between these bend-stretch levels in ArAlH is found to be 89.2 cm-', or 41% of Dg1.37 In the present study, we have found that this energy difference equals 212.1 cm-' for KrAlH, or 57% of Dol. We thus see that the differences in the A' and A'' PES's are of similar magnitude relative to the average of these PES's for all three systems. As in ArA1H,37we find that the decay lifetimes for excited KrAlH(A) levels are smaller than for free AlH. This implies that the electronically excited states of both of these complexes decay botn radiatively and by electronic quenching. These measurements represent the first examples of van der Waals complexes in which this excited state nonradiative decay process occurs to a significant extent.
Acknowledgment. The authors gratefully acknowledge their theoretician colleagues Millard Alexander, Moonbong Yang, and Susan Gregurick for numerous conversations about van der Waals complexes of open-shell diatoms and for their previous collaborative computational studies of the isovalent ArBH and ArAlH complexes. This research was supported by the U.S. Air Force Office of Scientific Research, under Grant AFOSR91-0363, and the National Science Foundation, under Grant CHE-922308 1. References and Notes (1) Heaven, M.C.Annu. Rev. Phys. Chem. 1992, 43, 283. (2) Heaven, M. C. J. Phys. Chem. 1993, 97, 8567. (3) Berry, M. T.; Brustein, M. R.;Adamo, J. R.; Lester, M. I. J. Phys. Chem. 1988, 92, 5551. (4) Fawzy, W. M.; Heaven, M. C. J. Chem. Phys. 1988, 89, 7030. (5) Fawzy, W.M.; Heaven, M. C. J. Chem. Phys. 1990, 92, 909. (6) Schleipen, J.; Nemes, L.; Heinze, J.; ter Meulen, J. J. Chem. Phys. Lett. 1990, 175, 56.
Spectroscopy of KrAIH(XIC+,AIII)Complex (7) Chang, B.-C.; Yu, L.; Cullin, D.; Rehfuss, B.; Williamson, J.; Miller, T. A.; Fawzy, W. F.; Zheng, X.; Fei, S.; Heaven, M. J. Chem. Phys. 1991, 95, 7086. (8) Chang, B.-C.; Williamson, J. M.; Cullin, D. W.; Dunlop, J. R.; Miller, T. A. J. Chem. Phys. 1992, 97, 7999. (9) Endo, Y.; Kohguchi, H.; Ohshima, Y. Faraday Discuss., in press. (10) Berry, M. T.; Brustein, M. R.; Lester, M. I. J. Chem. Phys. 1989, 90, 5878. (11) Berry, M. T.; Brustein, M. R.; Lester, M. I. J. Chem. Phys. 1990, 92, 6469. (12) Lester, M. I.; Randall, R. W.; Giancarlo, L. C.; Choi, S. E. J. Chem. Phys. 1993, 99, 6211. (13) Lester, M. I.; Choi, S. E.; Giancarlo, L. C.; Randall, R. W. Faraday Discuss.,in press. (14) Giancarlo, L. C.; Randall, R. W.; Choi, S. E.; Lester, M. I. J. Chem. Phys. 1994, 101, 2914. (15) Berry, M. T.; Brustein, M. R.; Lester, M. I.; Chakravarty, C.; Clary, D. C. Chem. Phys. Lett. 1991, 178, 301. (16) Berry, M. T.; Loomis, R. A.; Giancarlo, L. C.; Lester, M. I. J. Chem. Phys. 1992, 96, 7890. (17) Bowman, J. M.; Gazdy, B.; Schafer, P.; Heaven, M. C. J. Phys. Chem. 1990, 94, 2226, 8858. (18) Schnupf, U.; Bowman, J. M.; Heaven, M. C. Chem. Phys. Lett. 1992. 189. _, . . , 481. (19) Lester, M. I.; Loomis, R. A.; Giancarlo, L. C.; Berry, M. T.; Chakravarty, C.; Clary, D. C. J. Chem. Phys. 1993, 98, 9320. (20) Dubernet, M.-L.; Hutson, J. M. J. Chem. Phys. 1993, 99, 7477. (21) Degli Esposti, A,; Werner, H.-J. J. Chem. Phys. 1990, 93, 3351. (22) Dubernet, M.-L.; Flower, D.; Hutson, J. M. J. Chem. Phys. 1991, 94, 7602. (23) Green, Jr., W. H.; Lester, M. I. J. Chem. Phys. 1992, 96, 2573. (24) Dagdigian, P. I.; Alexander, M. H.; Liu, K. J. Chem. Phys. 1989, 91, 839. (25) Alexander, M. H.; Dagdigian, P. J. J. Chem. Phvs. 1994,101,7468. (26) Lemire, G. W.; McQGaidrM. J.; Kotlar, A. J.; Sausa, R. C. J. Chem. Phys. 1993, 99, 91. (27) Dubernet, M.-L.; Tuckey, P. A.; Hutson, J. M. Chem. Phys. Lett. 1992, 193, 355. (28) Alexander, M. H.; Gregurick, S.; Dagdigian, P. J.; Lemire, G. W.; McQuaid, M. J.; Sausa, R. C. J. Chem. Phys. 1994, 101, 4547. (29) Randall, R. W.; Chuang, C.-C.; Lester, M. I. Chem. Phys. Lett. 1992, 200, 113.
J. Phys. Chem., Vol. 99, No. 11, 1995 3437 (30) Jansen, G.; Hess, B. Chem. Phys. Lett. 1992, 192, 21. (31) Yang, M.; Alexander, M. H.; Chuang, C.-C.; Randall, R. W.; Lester, M. I. J. Chem. Phys., submitted. (32) Yang, M.; Alexander, M. H.; Werner, HA.; Hohmann, J.; Neitsch, L.; Stuhl, F.; Dagdigian, P. J. J. Chem. Phys., in press. (33) Hwang, E.; Dagdigian, P. J. J. Chem. Phys. 1994, 101, 2903. (34) Alexander, M. H.; Gregurick, S.; Dagdigian, P. J. J. Chem. Phys. 1994, 101, 2887. (35) Holmgren, S . L.; Waldman, M.; Klemperer, W. J. Chem. Phys. 1977, 67, 4414. (36) Yang, M.-C.; Salzberg, A. P.; Chang, B.-C.; Carter, C. C.; Miller, T. A. J. Chem. Phys. 1993, 98, 4301. (37) Hwang, E.; Dagdigian, P. J. J. Chem. Phys., in press. (38) Yang, M.; Gregurick, S.; Alexander, M. H.; Dagdigian, P. J. J. Chem. Phys., in press. (39) Hwang, E.; Huang, Y.-L.; Dagdigian, P. J.; Alexander, M. H. J. Chem. Phys. 1993, 98, 8484. (40) Zhang, Y.; Stuke, M. J. Cryst. Growth 1988, 93, 143. (41) Taken from Material Safety Data Sheet (Johnson Matthey Alfa Aesar, Ward Hill, MA). (42) Dagdigian, P. J.; Forch, B. E.; Miziolek, A. W. Chem. Phys. Lett. 1988, 148, 299. (43) Chen, J.; Dagdigian, P. J. J. Chem. Phys. 1992, 96, 7333. (44)Herzberg, G. Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules; D. Van Nostrand: Princeton, 1950. (45) Zeeman, P. B.; Ritter, G. J. Can. J. Phys. 1954, 32, 555. (46) American Institute of Physics Handbook; McGraw-Hill: New York, 1972. (47) Fu, Z.; Massick, S.; Kaup, J. G.; Benoist d'Azy, 0.;Breckenridge, W. H. J. Chem. Phys. 1992, 97, 1683. (48) Fei, S.; Zhang, X.; Heaven, M. C. J. Chem. Phys. 1992,97, 1655. (49) Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; D. Van Nostrand: Princeton, 1967. (50) Baltayan, P.; Nedelec, 0. J. Chem. Phys. 1979, 70, 2399. (51) Fraser, G. T.; Pine, A. S. J. Chem. Phys. 1986, 85, 2502. (52) Hutson, J. M.; Howard, B. J. Mol. Phys. 1982, 45, 769.
JP9424973
