Radiative and Nonradiative Decay of the BH(b3Σ-) State: A Joint

J. Phys. Chem. 1996, 100, 5649-5653

5649

Radiative and Nonradiative Decay of the BH(b3Σ-) State: A Joint Experimental and Theoretical Study Xin Yang, Lisa Pederson, David R. Yarkony, and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218-2685 ReceiVed: October 30, 1995; In Final Form: January 9, 1996X

Lifetimes of rotational/fine-structure levels of electronically excited BH(b3Σ-), in vibrational levels V′ ) 0-4, were determined from fluorescence decay waveforms with laser excitation on isolated rotational lines in the b3Σ--a3Π ∆V ) 0 sequence. BH was prepared in a pulsed supersonic beam by 193 nm photolysis of diborane. The measured lifetimes were compared with values obtained from a theoretical treatment of the excited state dynamics, in which both radiative decay to the a3Π state and nonradiative decay through the repulsive 13Σ+ state were considered. The experimental and theoretical lifetimes agree very well. This comparison shows that the low b3Σ- rotational levels for V′ ) 0-2 decay purely radiatively, with a rate decreasing as V′ increases. These rates are virtually independent of the fine-structure level. However, the lifetimes of the higher vibrational levels exhibit significant fine-structure level dependence, with the shortest values obtained for the F2 levels. This behavior is shown to be attributable to predissociation induced by b3Σ-∼13Σ+ spin-orbit coupling.

1. Introduction The BH molecule has evoked interest, in part because it has only six electrons and hence is amenable to high-accuracy electronic structure calculations. A considerable number of electronic states of the BH molecule have been spectroscopically identified in investigations spanning several decades; these studies have been reviewed recently.1 The strongest transition in the spectrum of the BH molecule is the well-characterized A1Π-X1Σ+ band system, recently recorded at high resolution by Fernando and Bernath.1 Current interest in the BH molecule is focused on the lowlying a3Π electronic state, whose minimum is calculated2 to lie 10 600 cm-1 above the bottom of the X1Σ+ potential energy curve. Boehmer and Benard3-5 have demonstrated that resonant energy transfer from metastable NF(a1∆) to BH can form the basis for a collisionally pumped high-gain visible wavelength chemical laser, emitting on BH A1Π f X1Σ+ lines. The pumping mechanism for this system involves two near-resonant spin-allowed collisional energy transfer steps through the a3Π state:

NF(a1∆) + BH(X1Σ+) f BH(a3Π) + NF(X3Σ-)

(1)

NF(a1∆) + BH(a3Π) f BH(A1Π) + NF(X3Σ-)

(2)

In recent experiments, Boehmer and Benard5 have monitored the time-dependent concentrations of the BH(X1Σ+,a3Π) states by absorption spectroscopy. The latter was observed through its near-UV transition to the b3Σ- state.6 Formation of the b3Σstate in the B(4p 2P) + H2 reaction has also been detected through BH chemiluminescence emission in the b3Σ--a3Π band system.7 The rotational lines of a number of bands in several sequences of this electronic transition have also recently been measured in emission at high-resolution through Fourier transform spectroscopy, and accurate spectroscopic constants covering a range of vibrational levels in the upper and lower states have been derived.8 X

Abstract published in AdVance ACS Abstracts, March 1, 1996.

0022-3654/96/20100-5649$12.00/0

In addition to the spectroscopic constants, another important set of parameters characterizing an electronic transition are the radiative transition probabilities, and, if the state predissociates, the nonradiative decay rates of individual rotational/finestructure levels in the upper electronic state. The former parameters are, of course, required for the calculation of band oscillator strengths, which can be used to relate measured absorbances to concentrations. In BH, predissociation of the b3Σ- state occurs because of spin-orbit coupling to the repulsive 13Σ+ state, which correlates with the ground atomic B(2p 2P) + H(1s 2S) asymptote. Pederson and Yarkony2,9 have calculated the radiative and nonradiative decay rates for rovibrational/fine-structure levels in the b3Σ- state, based on ab initio electronic structure data, including potential energy curves, electronic transition moments, and spin-orbit interactions. They concluded that nonradiative decay should have a measurable effect on the b3Σ- decay lifetimes for V′ g 3, and for high rotational levels in V′ ) 2. In the present paper, we report experimental measurements of decay lifetimes for rotational/fine-structure levels of BH(b3Σ-) in vibrational levels 0 e V′ e 4. These were determined by analysis of fluorescence decay waveforms obtained with laser fluorescence excitation of isolated lines in the b3Σ--a3Π band system of BH in a supersonic beam. The experimental lifetimes are compared with calculations of the total decay lifetimes for these specific levels. Calculations extending beyond the range of the experimental data are also reported. In the course of the comparison, we found that the previously reported9 b3Σ- f a3Π radiative transition probabilities were incorrectly calculated by a factor of 2, because of an unfortunate neglect of an electronic degeneracy factor. In addition, the nonradiative decay rates reported previously9 had the entries for the F1 and F3 finestructure levels interchanged. These errors have been corrected in this paper. Excellent agreement of experimental and computed decay lifetimes is found. Evidence for predissociation can be seen in the experimental lifetimes for small N′ and V′ > 2, in agreement with the theoretical predictions of Pederson and Yarkony.2 Despite the small magnitude of the b3Σ--13Σ+ coupling, predissociation may cause appreciable reduction in the b3Σ- decay lifetimes. With the present results, accurate © 1996 American Chemical Society

5650 J. Phys. Chem., Vol. 100, No. 14, 1996

Yang et al.

information from both experiment and theory is now available for the conversion of absorbances measured in the BH b3Σ-a3Π electronic transition into a3Π concentrations. 2. Experimental Section The apparatus in which the BH in its a3Π electronic state was prepared and promoted by laser irradiation to the b3Σ- state has been described in detail previously, in connection with our on-going spectroscopic studies of van der Waals complexes of B atoms, BH(X1Σ+), and AlH(X1Σ+).10-13 A brief description is presented here. A mixture of diborane, helium, and argon (typically 6-8 atm, of composition 0.7% diborane, 80% argon, and the remainder helium) was expanded into vacuum through a 0.2 mm diameter nozzle orifice by means of a pulsed solenoid valve (General Valve Series 9). BH molecules were generated in the early stages of the supersonic expansion by 193 nm photolysis of diborane with a focused, attenuated beam from an ArF excimer laser (Lambda Physik EMG101MSC) and were detected 1.2 cm downstream of the nozzle by laser fluorescence excitation with a dye laser (Lambda Physik LPD3002E). The dye laser was fired at a variable delay after the photolysis pulse to allow for the transit time from the photolysis to the detection zones. In order to discriminate against background light induced by the excimer laser, the dye laser induced fluorescence signal was collected with a telescope, passed through a color filter and a 1/ m monochromator, and detected with a photomultiplier (EMI 4 9813QB). In addition, the gain of the photomultiplier was switched off during the excimer laser pulse by zeroing the voltage difference between the photocathode and first dynode. For collection of fluorescence excitation spectra, the output of the photomultiplier was directed to a gated integrator and then to a laboratory computer. To collect the fluorescence decay waveforms for the measurement of excited state lifetimes, the photomultiplier output was passed to a digital oscilloscope (Tektronix 2430A). Some details about the specifications of the oscilloscope are relevant. The bandwidth of the analog section of this instrument is 125 MHz, and the digital sampling rate is 100 megasamples s-1, or 10 ns between channels. In our experiments, we utilized an interpolation feature for repetitive signals, which allowed digitization on a finer grid to achieve the full analog bandwidth. The waveform from an individual laser shot was obtained with the usual 10 ns spacing; however, waveforms from successive shots were taken with slightly shifted delays, controlled internally by the oscilloscope. The noise on the signal is then mainly the result of the shot-to-shot fluctuation in the signal. To improve the signal-to-noise ratio, waveforms were usually acquired using typically a 128-shot running average. Fluorescence decay waveforms were accumulated under computer control, and the data were stored on magnetic media for later analysis. The calibration of the oscilloscope time base was checked with a square wave pulse from a signal generator whose frequency was measured with a counter. 3. Spectra and Measured Lifetimes Figure 1 presents a laser fluorescence excitation spectrum of the (0,0) band of the BH b3Σ--a3Π transition. Because of the low rotational temperature of the beam, the strongest lines in the spectrum involve transitions out of the lowest rotational/ fine-structure level in the a3Π state, i.e., the J ) 0 F3 e/f Λ doublets. These strong lines access the J′ ) 1 fine-structure levels in the N′ ) 0, 1, and 2 rotational manifolds of the b3Σstate. Beams generated with seed gases of various heliumargon composition were investigated; the lowest rotational

Figure 1. Laser fluorescence excitation spectrum of a free jet containing BH molecules, produced by 193 nm photolysis of diborane, in a helium-argon seed gas mixture at a total backing pressure of 7 atm. A scan of the BH b3Σ--a3Π (0,0) band is displayed. Rotational lines which involve transitions out of the lowest rotational level (N ) 0, J ) 1, F3) are identified.

temperatures were found for large mole fractions of argon. In addition to the (0,0) band, we observed higher members of the ∆V ) 0 sequence of the b-a transition, up to V ) 5. This indicates that the BH(a3Π) molecules in the supersonic beam have considerable vibrational excitation. This contrasts sharply with our earlier observations11 on ground state BH(X1Σ+) molecules in similar supersonic beams. For the ground electronic state, most of the molecules were found to be in the V ) 0 vibrational level. We also find that the fluorescence intensity for excitation of the b-a bands is much smaller than for the A-X bands. Since the oscillator strengths for the b-a ∆V ) 0 bands will be shown below to be similar to those of the corresponding A-X bands,14 we conclude that the majority of BH molecules in the beam are in the ground X1Σ+ electronic state. The considerable vibrational excitation observed in the a3Π state is presumably due to a combination of the dynamics of photofragmentation of the diborane precursor and collisional vibronic relaxation in the supersonic beam expansion. Fluorescence decay waveforms were obtained for excitation of all three J′ ) 1 levels [N′ ) 0 F1, N′ ) 1 F2, and N′ ) 2 F3] in the vibrational levels V′ ) 0-4 of the b3Σ- electronic state. The fluorescence signals for excitation of V′ ) 5 were too weak to obtain analyzable decay waveforms. The J′ ) 1 F1, F2, and F3 levels were prepared by laser excitation on the R13(0), R23(0), and R33(0) lines, respectively, of the appropriate band in the ∆V ) 0 sequence. Figure 2 displays a typical waveform of the decay of a BH(b3Σ-) V′ ) 0 level. For some of the decay scans, particularly those involving the higher V′ levels, there was a noticeable contribution from scattered laser light. This contribution was eliminated or ignored, either by taking differences of waveforms with the photolysis laser on vs off or by discarding the first portion of the decay waveform. Table 1 presents the measured BH(b3Σ-) decay lifetimes and total decay rates (reciprocal of the lifetime) for the excited levels studied. The lifetimes were determined through weighted linear least-squares fits of the logarithm of the signal vs time. To avoid the contribution from the laser scattered light and the digitization error for small signals, the fits were restricted to signal levels between 85 and 10% of the maximum amplitude. The reported error estimates were obtained from the standard deviations of the fits to individual decay waveforms. Because of the low density of the molecular beam at the probe laser excitation zone (ca. 10-4 Torr), removal of excited molecules by collisional quenching can be neglected here. Moreover, flyout of the molecules from the detector field of view can be ignored since the distance traveled by a molecule in several radiative lifetimes is very small. We see from Table 1 that the measured decay lifetimes increase with increasing V′, for V′ e 2. As will be shown below,

Decay of the BH(b3Σ-) State

J. Phys. Chem., Vol. 100, No. 14, 1996 5651 tion.20 These data have been reported previously2,9 and will not be repeated here. The total radiative decay rates out of specific BH(b3Σ-) rotational levels were determined in a Hund’s case (a) basis21 from the electronic structure data using the following equations:

krad(b3Σ-,V′J′) ) ∑A(b3Σ-,V′J′;a3Π,V′′J′)

(3)

V′′

where

A(b3Σ-,V′J′;a3Π,V′′J′) ) 2.149 × 1010(hν)3µ(b3Σ-,V′J′;a3Π,V′′J′)2g(Σ,Π) (4)

Figure 2. Top panel: Fluorescence decay waveform (solid line) for excitation of the BH(b3Σ-) V′ ) 0, N′ ) 0, J′ ) 1 F1 level on the R13(0) line of the b3Σ--a3Π (0,0) band; the dashed line shows the scattered laser light signal. Lower panel: Semilogarithmic plot of the decay waveform.

TABLE 1: Measured and Calculated BH(b3Σ-) Lifetimes and Decay Rates measureda V′

N′, J′ Fi′

0

N′ ) 0, J′ ) 1 F1 N′ ) 1, J′ ) 1 F2 N′ ) 2, J′ ) 1 F3 N′ ) 0, J′ ) 1 F1 N′ ) 1, J′ ) 1 F2 N′ ) 2, J′ ) 1 F3 N′ ) 0, J′ ) 1 F1 N′ ) 1, J′ ) 1 F2 N′ ) 2, J′ ) 1 F3 N′ ) 0, J′ ) 1 F1 N′ ) 1, J′ ) 1 F2 N′ ) 2, J′ ) 1 F3 N′ ) 0, J′ ) 1 F1 N′ ) 1, J′ ) 1 F2 N′ ) 2, J′ ) 1 F3

1 2 3 4

τ (ns)

ktotb

calculated krad

kpredc

ktot

95 ( 3 10.5 ( 0.3 10.8 8.25(-7) 10.8 95 ( 3 10.5 ( 0.3 10.8 1.30(-6) 10.8 93 ( 3 10.8 ( 0.3 10.8 4.83(-7) 10.8 104 ( 3 9.6 ( 0.3 10.0 9.54(-4) 10.0 105 ( 3 9.5 ( 0.3 10.0 1.49(-3) 10.0 103 ( 3 9.7 ( 0.3 10.0 5.36(-4) 10.0 112 ( 3 8.9 ( 0.2 9.3 1.15(-1) 9.4 112 ( 3 8.9 ( 0.2 9.3 1.77(-1) 9.4 111 ( 3 9.0 ( 0.2 9.3 6.20(-2) 9.4 108 ( 4 9.3 ( 0.3 8.5 1.71 10.2 103 ( 4 9.7 ( 0.4 8.5 2.59 11.1 112 ( 4 8.9 ( 0.3 8.5 8.81(-1) 9.4 116 ( 6 8.6 ( 0.4 7.7 1.09 8.8 111 ( 5 9.0 ( 0.4 7.7 1.59 9.3 132 ( 6 7.6 ( 0.3 7.7 5.01(-1) 8.2

a Reported errors are 1σ estimates. b All decay rates given in units of 106 s-1. c Characteristic base 10 given parenthetically.

this behavior is due to a decrease in the b3Σ- f a3Π radiative decay rate with increasing V′. Above V′ ) 2, the lifetimes do not increase monotonically with increasing V′ and depend on the specific excited fine-structure level. The F2 levels have the shortest lifetimes. This behavior with the vibrational quantum number and fine-structure label provides experimental evidence for predissociation of the b3Σ- state. 4. Theoretical Treatment and Computed Radiative and Nonradiative Decay Rates A detailed discussion of our theoretical approach for the calculation of the BH(b3Σ-) radiative and nonradiative decay rates has been given previously.2,9 The requisite potential energy curves for the a3Π, b3Σ-, and 13Σ+ states, the a3Π-b3Σelectronic transition moment, and b3Σ- ∼ 13Σ+ spin-orbit interaction, were evaluated using state-averaged multiconfigurational self-consistent-field15-18/configuration interaction19 wave functions and extended contracted Gaussian basis sets. The spin-orbit interaction was evaluated within the Breit-Pauli approximation, including both the one-electron spin-orbit contribution and the two-electron spin-other orbit contribu-

In eq 4, hν is the energy gap between the upper and lower rovibronic levels, and µ(b3Σ-, V′ J′; a3Π, V′′ J′) is the electronic transition moment averaged over the appropriate solutions to the case (a) nuclear Schro¨dinger equation. The electronic degeneracy factor22 g(Σ,Π) in eq 4 equals 2; for all other combinations of Λ′ and Λ, g(Λ′,Λ) equals unity. This term was inadvertently omitted in our previous calculation9 of the radiative decay rates. Our neglect in eq 4 of the fine-structure dependence of the radiative decay rates is acceptable in view of the limited rotational dependence found in the calculated rates9 and is supported by the experimental data presented here. For the radiationless decay pathway, it is essential to distinguish among the fine-structure levels, as they exhibit different decay rates. The radiationless decay rates were determined in a Hund’s case (b) basis using the following Golden rule approximation:23,24

kpred(b3Σ-,V′N′Fi′) ) (4µ/k)C(3Σ-,N′Fi′;3Σ+)Hso(b3Σ-,V′N′;13Σ+,EN′)2 (5) where Hso(b3Σ-,V′N′;13Σ+,EN′) is the b3Σ-∼13Σ+ spin-orbit matrix element averaged over the appropriate solutions to the Hund’s case (b) nuclear Schro¨dinger equation. The factor C(3Σ-,N′Fi′;3Σ+) in eq 5, discussed in more detail in ref 9, equals (J′ + 1)/(2J′ + 1), 1, and J′/(2J′ + 1) for the fine-structure levels Fi′, i ) 1, 2, 3, respectively, where J′ is the total angular momentum for the particular N′, Fi′ combination (see Table 1). Equation 5 does not apply to the J′ ) 0 level since this level does not predissociate because of parity conservation. This perturbative treatment for the calculation of the predissociation rates is expected to be excellent in view of the weak b3Σ-13Σ+ spin-orbit interaction ( 2. The 13Σ+ potential energy curve is predicted to cross the outer limb of the b3Σ- curve slightly below the energy of the V′ ) 3

5652 J. Phys. Chem., Vol. 100, No. 14, 1996

Yang et al.

TABLE 2: Calculated BH(b3Σ-) Radiative and Nonradiative Decay Ratesa V′ N′ (or J′)b

Fi′

0

1

2

3

4

5

2

radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3 radiative F1 F2 F3

1.08(1) 7.55(-7) 1.40(-6) 4.83(-7) 1.08(1) 7.99(-7) 1.55(-6) 6.54(-7) 1.08(1) 8.83(-7) 1.77(-6) 8.17(-7) 1.07(1) 1.01(-6) 2.09(-6) 1.02(-6) 1.04(1) 3.25(-6) 7.50(-6) 4.21(-6) 9.92 1.92(-5) 4.76(-5) 2.90(-5) 9.26 1.61(-4) 4.20(-4) 2.68(-4) 8.44 1.52(-3) 4.03(-3) 2.62(-3) 7.49 1.31(-2) 3.43(-2) 2.22(-2)

1.00(1) 8.54(-4) 1.56(-3) 5.36(-4) 9.99 8.79(-4) 1.68(-3) 7.01(-4) 9.96 9.36(-4) 1.85(-3) 8.38(-4) 9.92 1.03(-3) 2.08(-3) 9.92(-4) 9.62 2.28(-3) 5.08(-3) 2.75(-3) 9.14 7.77(-3) 1.83(-2) 1.06(-2) 8.49 3.29(-2) 7.97(-2) 4.75(-2) 7.71 1.41(-1) 3.41(-1) 2.04(-1) 6.79 5.03(-1) 1.18 6.82(-1)

9.26 1.01(-1) 1.82(-1) 6.20(-2) 9.24 1.02(-1) 1.91(-1) 7.88(-2) 9.21 1.05(-1) 2.03(-1) 9.06(-2) 9.17 1.10(-1) 2.18(-1) 1.02(-1) 8.88 1.78(-1) 3.78(-1) 1.97(-1) 8.41 3.64(-1) 8.01(-1) 4.34(-1) 7.77 7.82(-1) 1.73 9.39(-1) 7.00 1.43 3.03 1.58 6.10 1.67 3.22 1.51

8.49 1.47 2.62 8.81(-1) 8.47 1.45 2.66 1.08 8.45 1.45 2.72 1.19 8.41 1.46 2.79 1.27 8.14 1.65 3.28 1.60 7.70 1.84 3.61 1.75 7.09 1.55 2.82 1.25 6.32 5.53(-1) 7.41(-1) 2.17(-1) 5.43 2.17(-2) 1.93(-1) 2.15(-1)

7.71 9.17(-1) 1.54 5.01(-1) 7.69 8.63(-1) 1.47 5.65(-1) 7.67 8.08(-1) 1.38 5.57(-1) 7.64 7.46(-1) 1.27 5.19(-1) 7.39 3.39(-1) 4.85(-1) 1.66(-1) 6.98 7.41(-3) 2.59(-3) 2.24(-2) 6.40 2.99(-1) 8.76(-1) 5.75(-1) 5.65 9.18(-1) 1.88 9.02(-1) 4.75 4.35(-1) 5.27(-1) 1.19(-1)

6.94 1.17 2.07 6.95(-1) 6.92 1.14 2.08 8.39(-1) 6.90 1.13 2.09 9.00(-1) 6.87 1.11 2.09 9.31(-1) 6.63 1.02 1.86 8.24(-1) 6.25 6.22(-1) 9.23(-1) 3.10(-1) 5.70 5.78(-2) 7.54(-3) 1.41(-2) 4.96 2.41(-1) 8.03(-1) 5.53(-1) 4.00 6.23(-1) 1.10 4.21(-1)

3

4

5

10

15

20

25

30

a Rates given in units of 106 s-1. Characteristic base 10 given parenthetically. b Radiative rates given as a function of the case (a) rotational angular momentum J′; nonradiative rates (for each fine-structure level Fi′) given as a function of the case (b) rotational angular momentum N′.

vibrational level.9 Despite the small magnitude of the b3Σ-13Σ+ spin-orbit matrix element, ∼0.5 cm-1 in the vicinity of the crossing,9 the predissociation rates for V ) 3 are as much as 30% of the radiative decay rate (see Table 2). As noted previously, the predissociation rate is the largest for F2 finestructure levels. These levels have the largest coupling to the dissociation continuum since, of the three manifolds of finestructure levels, the F2 levels have the largest Ω ) 1 character (100% Ω ) 1f), and the b3Σ--13Σ+ mixing occurs only for the Ω ) 1 sublevel in the nonrotating molecule. 5. Discussion and Conclusion The experimentally measured and theoretically predicted BH(b3Σ-) decay rates are compared in Table 1 and in Figure 3. It can be seen that the measured dependence of the lifetimes on the vibrational level V′ and the fine-structure level Fi′ is very well reproduced by the calculations. Note that the electronic structure data are used unmodified in the calculation of decay rates. In previous treatments of radiationless decay in firstrow hydrides, it was found necessary to adjust the position of the dissociative curves to obtain reliable results.25-28 The calculated purely radiative b3Σ- f a3Π decay rates are only 4% higher (average of the differences in the total decay rates for all V′ e 2 levels) than the experimental radiative decay rates. This difference is within our estimated experimental 2σ error bars. The radiative lifetime of a BH(b3Σ-) vibrational level V′ is shorter than the radiative lifetime of the corresponding vibrational level in the A1Π electronic state. For example, the

Figure 3. Comparison of the experimental and theoretical decay lifetimes of the BH(b3Σ-,V′N′Fi′) levels: (a) N′ ) 0 F1 levels, (b) N′ ) 1 F2 levels, (c) N′ ) 2 F3 levels. The experimental lifetimes are represented by solid circles, while the calculated values are denoted with open circles.

lifetime of the V′ ) 0 level of BH(A1Π) is reported14 to be 127 ( 10 ns, while we find a value of 95 ( 3 ns for the V′ ) 0

Decay of the BH(b3Σ-) State level of BH(b3Σ-). Adjusting for ν3 and electronic degeneracy factor g(Λ′,Λ) in eq 4, the strengths of the A-X and b-a electronic transitions are similar. This is not unexpected since in the molecular region the electronic transition moments µ(A,X) and µ(b,a) arise principally from nominally the same oneelectron σ r π transition moment integral,9,29 with the µ(A,X)/ µ(b,a) ) x2 at this level of approximation (see Table 4 of ref 2). Both the experimental and theoretical values of the BH(b3Σ-) lifetimes are seen to drop in going from V′ ) 2 to V′ ) 3, signifying the onset of significant predissociation in the excited state. Both the experimental and theoretical data show the largest effect of predissociation for the F2 levels. The differences between experimental and calculated V′ ) 3 lifetimes are somewhat greater than for the lower V′ levels. The lifetimes are longer for V′ ) 4 than for V′ ) 3. This reflects the less favorable overlap of the b3Σ- vibrational wave function with the continuum wave functions.9 Nevertheless, the V′ ) 4 lifetimes are depressed below those for purely radiative decay. The fact that the theoretical values reproduce well the vibrational dependence of the measured lifetimes for V′ ) 3 and 4 indicates that the electronic structure calculations accurately predict the internuclear separation at which the repulsive 13Σ+ curve crosses the outer limb of the b3Σ- potential energy curve. Since predissociation of an excited state depresses the fluorescence quantum yield Φ, this nonradiative decay process can often be identified from a reduction of rotational line intensities in an emission spectrum,29 although this is not always the case.26,27 The detection of b3Σ-∼13Σ+ coupling by this method would be difficult in the present situation since even the most predissociated level for which a decay lifetime was measured [V′ ) 3, N′ ) 1 F2] has Φ > 75%. Although this would have a significant effect on inferred concentration, emission intensities would have to be measured very accurately in order to observe this enhanced predissociation of the F2 levels. In summary, lifetimes for the decay of BH(b3Σ-) molecules in various rovibronic levels have been experimentally determined and compared with theoretical lifetimes calculated with electronic structure data. The agreement of experimental and theoretical values is excellent. The b3Σ- state is seen to be predissociated by weak spin-orbit coupling with the repulsive 13Σ+ state.

J. Phys. Chem., Vol. 100, No. 14, 1996 5653 Acknowledgment. This research was supported by the U.S. Air Force Office of Scientific Research under Grant Nos. F49620-95-1-0055 (to P.J.D.) and F49620-93-1-0067 (to D.R.Y.). We are grateful to C. R. Brazier for sending us data from his unpublished spectroscopic study, including a list of observed lines in the BH b3Σ--a3Π band system and the resulting derived spectroscopic parameters. References and Notes (1) Fernando, W. T. M. L.; Bernath, P. F. J. Mol. Spectrosc. 1991, 145, 392. (2) Pederson, L. A.; Hettema, H.; Yarkony, D. R. J. Phys. Chem. 1994, 98, 11069. (3) Benard, D. J.; Boehmer, E.; Michels, H. H.; Montgomery Jr., J. A. J. Phys. Chem. 1994, 98, 8952. (4) Benard, D. J.; Boehmer, E. Appl. Phys. Lett. 1994, 65, 1340. (5) Boehmer, E.; Benard, D. J. J. Phys. Chem. 1995, 99, 1969. (6) Almy, G. M.; Horsfall, R. B. Phys. ReV. 1937, 51, 497. (7) Yang, X.; Dagdigian, P. J. J. Phys. Chem. 1993, 97, 4270. (8) Brazier, C. R. J. Mol. Spectrosc., submitted. (9) Pederson, L. A.; Yarkony, D. R. Mol. Phys. 1995, 84, 611. (10) Hwang, E.; Huang, Y.-L.; Dagdigian, P. J.; Alexander, M. H. J. Chem. Phys. 1993, 98, 8484. (11) Hwang, E.; Dagdigian, P. J. J. Chem. Phys. 1994, 101, 2903. (12) Hwang, E.; Dagdigian, P. J. J. Chem. Phys. 1995, 102, 2426. (13) Yang, X.; Hwang, E.; Dagdigian, P. J.; Yang, M.; Alexander, M. H. J. Chem. Phys. 1995, 103, 2779. (14) Douglass, C. H.; Nelson, H. H.; Rice, J. K. J. Chem. Phys. 1989, 90, 6940. (15) Docken, K.; Hinze, J. J. Chem. Phys. 1972, 57, 4928. (16) Hinze, J. J. Chem. Phys. 1973, 59, 6424. (17) Werner, H.-J.; Meyer, W. J. Chem. Phys. 1981, 74, 5794. (18) Lengsfield, B. H. J. Chem. Phys. 1982, 77, 4073. (19) Shavitt, I. In Modern Theoretical Chemistry; Schaefer, H. F., Eds.; Plenum Press: New York, 1976; p 189. (20) Yarkony, D. R. Int. ReV. Phys. Chem. 1992, 11, 195. (21) Lefebvre-Brion, H.; Field, R. W. Perturbations in the Spectra of Diatomic Molecules; Academic Press: New York, 1986. (22) Larsson, M. Astron. Astrophys. 1983, 128, 291. (23) Rice, O. K. Phys. ReV. 1929, 33, 748. (24) Rice, O. K. Phys. ReV. 1929, 34, 1451. (25) Patel-Misra, D.; Parlant, G.; Sauder, D. G.; Yarkony, D. R.; Dagdigian, P. J. J. Chem. Phys. 1991, 94, 1913. (26) Parlant, G.; Dagdigian, P. J.; Yarkony, D. R. J. Chem. Phys. 1991, 94, 2364. (27) Bohn, B.; Stuhl, F.; Parlant, G.; Dagdigian, P. J.; Yarkony, D. R. J. Chem. Phys. 1992, 96, 5059. (28) Yarkony, D. R. J. Chem. Phys. 1992, 97, 1838. (29) Herzberg, G. Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules; D. Van Nostrand: Princeton, NJ, 1950.

JP9531873