Vibrational predissociation and electronic quenching dynamics of OHN2 (A2Σ+)

23 June 1995

CHEMICAL PHYSICS LETTERS Chemical Physics Letters240 (1995) 1-9

ELSEVIER

Vibrational predissociation and electronic quenching dynamics of OH-N 2 (A 2 ÷) Leanna C. Giancarlo, Marsha I. Lester Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA

Received 27 March 1995; in final form 19 April 1995

Abstract The vibrational predissociation and electronic quenching dynamics of weakly bound OH-N z (A 2X +) complexes have been examined using laser-induced fluorescence and dispersed fluorescence. We find that predissociation occurs on a ~< 1 ps timescale for complexes prepared in levels correlating with OH A2X+(v ' = 1)+ N2 X lX+(v"= 0). Furthermore, we suggest a simple kinetic analysis which describes the removal of population and lack of fluorescence from OH-N 2 levels bound by the OH A 2X +(v, = 0) + N2 X 1~ +(v,, = 0) potential energy surface. This mechanism implies that nonadiabatic decay, i.e. quenching, may be important for these complexes.

1. Introduction

N 2 with a cross section for VET, trVET, equal to 24.7

Collisional deactivation (quenching) of electronically excited atoms or molecules by ground state partners has received much attention over the years. Due to its importance in atmospheric chemistry and combustion processes, the OH radical has been the subject of much of this interest. For example, Lengel and Crosley [1,2] have pioneered this work with extensive studies on the quenching of OH A 2~÷ fluorescence by such collision partners as Ar, H2, and N 2. These investigations have sought to elucidate a mechanism for quenching by analyzing the removal of population (decrease in spontaneous emission) from an initially prepared OH A2X ÷ rovibrational level due to collisions with another partner. They have determined cross sections for both quenching from OH ( v ' = 0) and vibrational energy transfer (VET) from OH (v' = 1) by nitrogen: OH efficiently transfers its vibrational excitation to

•~° 2, while a smaller but sizeable cross section for quenching of OH ( v ' = 0), tro0, of 5.8 ,~z was obtained [1,2]. troo and trVET for N 2 are both considerable when compared to the same cross sections obtained for collisions between OH A 22~+ and Ar: CrvEx has been determined to be 0.42 ,~2 for Ar, and troo has been found to be negligible [2,3]. The authors have proposed that the significant O%Ex for OH A 2 ~ + ( J = 1) with N 2 is due to the large number of N 2 internal degrees of freedom (in both rotation and vibration) which are capable of accepting the OH vibrational energy. Subsequent studies by Copeland et al. [4] have probed the rotational level dependence of quenching from OH A 2~ + (v, = 0); as OH is prepared in higher initial rotational levels (n'), the quenching cross section with N 2 has been found to decrease. These studies have yielded an improved value for troo, when looking at OH A 2 E + ( v ' = 0, n' = 0) + N2, of

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L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9 0 2

6.7 A . The more recent work of Copeland, Wise and Crosley [5] has separated o-ol, the quenching cross section for OH A 2 E + ( v ' = 1), from the collision cross section for total removal from that state (0"% + O'VET). A slightly higher value for 0-VEV of 36 A2 has been attained, while O-ol has been determined to be negligible within experimental error limits. It should be noted that all of these deactivation studies rely on the fact that the OH A 2 ~ + molecules are randomly oriented with respect to the collision partner and that 0-00, 0-Ol, and O'VEr are thus averaged over all orientations of the collision pair. In addition, quenching in the full collision samples a thermal distribution of velocities (at 300 K). A necessary extension of these studies is the determination of the quenching mechanism and VET analog when OH and N 2 are brought together under the restricted geometry conditions of a weakly bound complex, O H - N 2. While there is now much experimental and theoretical information on open-shell diatom-rare gas systems (OH-Ar [6,7], OH-Ne [8-10]) and closedshell diatom-diatom systems (N2-HF [1-17]), there have been only limited investigations of open-shell diatom-diatom systems (HF-NO [18]) to date. In this Letter we report, for the first time, the detection of O H - N 2 (A 22~+) complexes by laser-induced fluorescence. On average, the O H - N 2 (X 2II) system is expected to be similar to the Nz-HF system by analogy with OH-Ar (X 2II) and the corresponding closedshell Ar-HF; specifically, OH-Ar and A r - H F have similar ground state binding energies ( = 100 cm-1), bond lengths (3.65 ,~), and minimum energy structures (linear configurations) [6]. O H - N 2 (X 2II) will differ from N2-HF, however, in that two different potentials correlate with N 2 + OH X 21/ depending on the orientation of the unpaired electron with respect to the O H - N 2 plane [19]. Microwave spectroscopy has been used by Flygare and co-workers [11] to discern the following about the N2-HF complex: a linear ground state geometry, N---N-HF, an intermolecular bond distance (R'~) of 3.5756 ,~ for the lowest intermolecular level, and an estimate for the ground state well depth (D~' = 618 cm- 1). Intermolecular vibrational levels of the N2-HF complex correlating with N 2 X 1 E + + H F X 1 E + ( v = 0 , 1)

have been examined using near infrared spectroscopy by Lisy [12], Nesbitt [13,14], Miller [15] and co-workers. Recently, Bemish et al. [16,17] have performed an elegant two-laser experiment to obtain the HF and N 2 photofragment vibrational, rotational, and translational distributions following vibrational predissociation of the N2-HF (v = 1) complex. These experiments have enabled the authors to determine that the complex dissociates to yield fragments in two distinct, highly selective product channels: N 2 (v = 1), HF (v = 0, j = 7) and N 2 ( v = 0), HF (v = 0, j = 12). HF is produced in the highest energetically available rotational level when N 2 is released with and without vibrational excitation; further, the vibrationally excited N 2 results from a V - V transfer of the initially excited HF vibrational energy across the weak intermolecular bond into the N 2 molecule. From fits of the angular distributions of the photoproducts, Bemish et al. [16] have also been able to establish a ground state binding energy for N2-HF of 398 cm- 1. In this Letter we explore the predissociation dynamics of weakly bound O H - N 2 complexes prepared in levels correlating with OH A 2~+(v' = 1) + N 2 X 1E + (v, = 0). These dynamics are discussed in light of the recent work performed on Nz-HF [16,17] and earlier collision studies [1-5]. Further, we develop a simple kinetic analysis to describe the removal of population and, thus, the lack of fluorescence from O H - N 2 intermolecular levels bound by the OH A 2E+(v' = 0) + N 2 potential energy surface. The latter suggests that a nonadiabatic decay mechanism, quenching, is important in the complex as well as in the collision.

2. Experimental Laser-induced fluorescence (LIF) and dispersed fluorescence have been used to investigate O H - N 2 complexes in the vicinity of the OH A 2E+-X 2II 1-0 and 0 - 0 transitions. The method used to produce the O H - N 2 complexes is similar to that used in this laboratory to prepare OH-Rg (Rg = Ne, Ar) and has been described elsewhere [6,10,20,21]. Briefly, OH radicals are generated in a supersonic expansion of HNO 3 in 20% nitrogen/argon carrier gas (backing pressure = 60 psi). The nitric acid is photolyzed

L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9

using the 193 nm output of an ArF excimer laser just downstream from the exit of a quartz capillary (1.2 cm length; 0.30 mm bore) attached to a pulsed valve. For studies on O H - N 2 in the OH A - X 1-0 region (278-282 nm), the frequency doubled output of a Nd : YAG-pumped dye laser with rhodamine 590 dye is employed to investigate the complexes approximately 1.5 cm from the end of the quartz capillary. The O H - N 2 complexes are probed in the OH A - X 0 - 0 region between 305 and 308 nm using sulforhodamine 640 dye. The frequency double dye laser resolution is ~ 0.2 cm -1 . The resultant fluorescence is collected with a blue sensitive photomultiplier tube (EMI 9813Q) positioned perpendicular to both the molecular beam and probe laser axes. A bandpass filter, centered at 310 nm (fwhm = 11 nm), passes OH 0 - 0 and 1-1 emission but blocks 1-0; a 193 nm (WG295) filter is used to block out background light, and the photomultiplier tube (PMT) is gated off during photolysis. All of the recorded spectra have been calibrated to the absolute frequencies of nearby OH rotational lines [22]. Dispersed fluorescence experiments are conducted by positioning a 0.25 m monochromator between the collection optics and the photomultiplier tube; an f / 4 lens is matched to the monochromator versus the f / l lens used with the PMT alone. The monochromator is scanned over the OH 1-1, 0-0, and 1-0 spectral regions around 314 nm (31860 cm 1), 308 nm (32440 cm-1), and 282 nm (35429 cm-1), respectively. The resolution of the monochromator is approximately 60 cm-~ and enables us to distinguish between emission from the complex (1-1 or 1-0 region) and OH A 2 ~ + fragments resulting from vibrational predissociation (0-0 region). The step size of the monochromator is approximately 45 cm- ~. The wavenumber of the laser scatter in the OH 1-0 region is used to calibrate the dispersed fluorescence spectra. A pressure and concentration dependence study has also been performed on the O H - N 2 features identified in the OH A - X 1-0 region. The concentration of N 2 gas in Ar has been varied in aliquots of 10%, 20%, and 100% as well as 10% and 25% in first run Ne. The best O H - N 2 (A 2~+) complex signals are obtained when 20% N 2 in Ar is used as the carrier gas; when pure nitrogen is employed, the complex signals decrease by more than a factor of 2,

3

implying that higher order complexes are being formed and depleting the binary O H - N 2 complexes probed in the LIF experiments. For the pressure dependent study, the backing pressure of the 20% N2/Ar carrier gas has been changed from 30 to 90 psi in 15 psi increments. The signals from OH-N 2 increase from 30 to 60 psi and then begin to decrease. The N2/Ar concentrations used to generate OH-N 2 complexes are similar to the conditions reported for N2-HF production, where rotationally resolved spectra have been obtained [13,16].

3. Results and analysis

3.1. Laser-induced fluorescence Fluorescence excitation spectra of O H - N 2 have been collected in the OH A 2 ~ +_X 2H 1-0 and 0 - 0 spectral regions. A representative spectrum from the OH 1-0 region is depicted in Fig. 1. We have identified three features, labelled A, B, and C, which can be attributed to the O H - N 2 complex; other sharp lines in this spectrum are rovibrational transitions due to excitation of the bare OH diatom. The three broadened O H - N 2 A, B, and C bands appear at

RI(5)

SR21(I )

SR21(2)

SR:l{3 )

B

35550

35600

35650

35700

Wavenumbers (cm" I )

Fig. 1. Fluorescence excitation spectrum of O H - N 2 in the OH A 2 ~ + - X 2H 1-0 region. Features due to excitation of intermolecular vibrations of the O H - N 2 complex at 35580.0, 35649.3 and 35710.5 c m - I are labelled A, B, and C, respectively. The large homogeneous linewidths (13.0 cm-1 for features A and B and 5.0 cm-~ for feature C) are attributed to the fact that these O H - N z complexes undergo predissociation. The sharp lines present in the spectrum are assigned as OH rovibrational transitions.

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L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9

Table 1 Positions, homogeneous linewidths, and lifetimes of OH-N 2 (A 2X +) features OH-N 2 feature

Position (cm - 1) a

Linewidth (cm- 1)

Lifetime (ps) d

A B C

35580.0 35649.3 35710.5

13.0 b 13.0 b 5.0 c

0.4 0.4 1.0

a Position reflects the band origin as derived from a rotational band contour analysis. Error bars are ± 0.1 cm-1. b Linewidths are taken from the Lorentzian component deconvoluted from a Voigt line profile. Uncertainty is ± 1.0 cm- 1. c Linewidths are taken from the Lorentzian component deconvoluted from a Voigt line profile. Error bars are + 0.5 cm- 1. d Lifetimes (7) are determined using the Heisenberg uncertainty relationship, ~'F= h / 2 ~r, where F is the homogeneous component of the linewidth.

excitation wavenumbers of 35580.0, 35649.3, and 35710.5 cm -~, respectively, and correspond to preparation of the complex in intermolecular vibrational levels correlating with OH A 2 E + ( v ' = 1) + N 2 X 1,2,÷ ( v " = 0). A fourth, much weaker feature has been observed around 35486 c m - 1 ; spectral congestion due to overlapping OH rovibrational lines precludes a detailed analysis of this band. Each of the O H - N 2 features, A, B, and C, extends over 30 c m - 1 and lies to higher energy (by as much as 280 cm - 1 ) above the OH Pl(1) transition at 35429.14 cm -1. This shift to higher energy indicates that the intermolecular levels identified in the excited electronic state are less well bound than the lowest level in the ground state, as discussed below. L I F spectra have also been recorded when exciting O H - N 2 in the vicinity of the OH A - X 0 - 0 transition. Although O H - N 2 features corresponding to those observed in the 1 - 0 region are expected to be seen in this region, specifically between 32590 and 32725 c m - 1 , none have been detected by LIF. Two features have been observed, however, to much higher energy, approximately 400 c m - 1 above OH PI(1), at 32823 and 32859 c m - t ; these features are probably due to internal rotational predissociative resonances [21] and will not be discussed here. The lack of features in the 32590 to 32725 cm -~ range suggests that a nonadiabatic decay channel removes O H - N 2 population from its excited electronic state on a timescale which is rapid compared to the radiative lifetime of the O H - N 2 complex (rrad -- 700 ns) [23,24]. A rotational band contour analysis has been performed for each of the O H - N 2 features identified in the OH 1 - 0 region. This analysis takes into account t h e O H - N 2 band origin, estimates for the ground

and excited state rotational constants based on N 2 H F 1, and homogeneous lifetime broadening derived from the Lorentzian component of the linewidth assuming a Voigt line profile; a laser bandwidth of 0.2 c m - 1 and a rotational temperature of 8 K have been used. The positions, linewidths, and lifetimes of features A, B, and C are presented in Table 1. All of the O H - N 2 features are rather broad and lack resolvable rotational structure. For instance, the homogeneous component to the linewidth of features A and B, which may have contributions from both vibrational predissociation and quenching, is 13.0 ___ 1.0 c m - 1 ; this corresponds to a lifetime of 0.4 ps. As will be shown later in this paper, the fluorescence is derived from OH A 2 ~ + (v, = 0) photofragments. Features A and B have been simulated assuming a perpendicular E - I I transition; this perpendicular transition results in a strong Q branch relative to the P and R branches [7,25]. Feature C, however, is slightly sharper than either features A or B and is best fit using a parallel 17-I-I transition. For this transition type, the Q branch of the complex becomes much weaker in intensity than the P and the R branches [7,25]. The linewidth of feature C is 5.0 + 0.5 c m - 1 which yields a lifetime of 1.0 ps. Again, the emission detected arises from OH A 22~ +(v' = 0) fragments. The predissociation lifetimes (~-) listed in Table 1 for O H - N 2 ( A 2E+, v' = 1) are approximately a factor of 100 times shorter than the vibrational predisso-

1 B" has been taken to be 0.158 cm -1 based on a comparison with N2-HF (K~ = 3.5756 .~) [11]. B' has been allowed to vary. For features A and B, a value of 0.159 cm -1 has been used for B', while B' = 0.162 cm -1 for feature C.

L.C. Giancarlo, M.I. Lester / Chemical Physics Letters 240 (1995) 1-9

ciation lifetimes reported for O H - A t (A 2E +, v' = 1) intermolecular vibrational levels [6]. Like the O H - A t lifetimes, the O H - N 2 ~"'s increase with intermolecular excitation of the complex. Furthermore, we note that the observed increase in predissociation rate ( k = l / T ) for O H - N 2 as compared to OH-Ar is consistent with the ~ 100 times increase in trVET obtained for OH A 2X+(v' = 1) in collisions with N 2 (36 .~2) as compared to that for Ar (0.42 ~2) [2,5].

5

[~1 I I - I I I l l l l l l P ~ 131211109 8 7 6 54321 I I IIIII Q2 12 9 7 1 1210 7531 : 131211109 8 7 6 5 4 3 2 [ ~ 1

3 511

3.2. Dispersed fluorescence Since O H - N 2 complexes prepared in levels correlating with OH A 2E+(v' = 1) + N 2 X 1E+(v" = 0) may undergo vibrational predissociation, we have used dispersed fluorescence in order to ascertain whether the fluorescence we have observed when exciting features in the OH A - X 1-0 region derives from O H - N 2 complexes or OH A 2 ~ ÷ ( v ' = 0 ) photofragments. The spontaneous emission from the O H - N 2 A, B, and C bands has been dispersed using a 0.25 m monochromator, the dispersed fluorescence spectra are displayed in Fig. 2. These spectra have been recorded by scanning the monochromator over the OH A - X 1-1, 0-0, and 1-0 regions at steps of approximately 45 cm-1 per point with the slits set at 0.2 to 0.3 mm to achieve 60 cm -1 resolution. Despite the fact that this resolution does not enable us to resolve individual OH rotational lines, we can readily discern that the dispersed emission is peaked at 309 nm, corresponding to emission from the OH A - X 0 - 0 region. Moreover, the breadth of the dispersed features (up to 1000 cm -1) indicates that the fluorescence is emitted from OH A 2 X + ( v ' = O, d) photofragments populating many different rotational levels (n'). The positions of the OH A - X 0 - 0 rovibrational transitions are labelled with tic marks in Fig. 2. Since no fluorescence is detected from the undissociated complex (in the 1-1 or 1-0 regions around 314 and 282 nm, respectively), predissociation must occur on a much faster timescale than radiative decay, where the radiative lifetime, Trad, for O H - N 2 is assumed to be equal to that for OH (~ 700 as) [23,24]. Complexes excited to the levels at 35580 and 35649 cm -1 (features A and B) yield the same dispersed fluorescence spectra as shown by the solid line in Fig. 2. For these cases, OH rotational levels

31500

32000

32500

33000

Wavenumbers {cm"I )

Fig. 2. Dispersed fluorescence spectra following excitation of OH-N 2 in the OH A - X 1-0 region. The solid line is the dispersed emission obtained following excitation of OH-N 2 features A and B; the dashed line is the dispersed fluorescence collected when exciting feature C. The emission is due to OH A 2E ÷ fragments which are produced following vibrational predissociation of OH-N~ (A2• +, v'= 1). The labels indicate the OH rovibrational transitions which contribute to the dispersed fluorescence spectra. The monochromator resolution is 60 c m - l with a step size of 45 cm-1.

in the range from n' = 0 to 12 are populated with non-negligible population in n' = 8 through 12. When O H - N 2 is prepared in the level at 35710 cm -1 (feature C), however, the emission spectrum changes markedly as represented by the dashed line. Here, the OH population distribution is much narrower, extending only to n ' = 8. The differences between features A and B compared to feature C are therefore most pronounced in the high n' region of the dispersed fluorescence spectrum. Excitation of the complex to all three levels produces OH fragments in low rotational levels as evidenced by the position of the peak of the dispersed fluorescence spectrum which is centered around 32350 cm -1. Moreover, the fact that many OH rotor levels are populated suggests that the OH A 2 ~ + + N2 potential energy surface is highly anisotropic [20]. The dispersed fluorescence spectra have been simulated in order to obtain a qualitative estimate of the relative population in each OH A 2E + rotational

6

L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9

level following vibrational predissociation from an initially prepared O H - N 2 intermolecular level. The simulations are performed by taking into account the H/Snl-London line strength factors for each OH transition and the resolution of the monochromator, while allowing the population in each OH rotational level to vary. The simulations reveal that most of the OH fragments ( > 50%) are produced with n' = 0; the population from n' = 0 to 2 decreases exponentially with negligible population in n' = 3 through 5 when O H N 2 complexes are excited to the levels at 35580 and 35649 cm-1 (features A and B). Small but sizeable population is found for OH rotor levels from n' = 6 to 12 ( = 20%). When O H - N 2 complexes are prepared in the highest energy intermolecular level (feature C at 35710 cm-1), the population distribution cuts off abruptly at n' = 8. There is substantial and nearly equal population in all OH rotor levels up through n' = 8. No population is found in OH (v' = 0) levels higher than n ' = 8 in the product distribution obtained for feature C, in sharp contrast to the distribution obtained when dispersing the emission from features A and B. The OH A 2£ + rotational levels which are energetically accessible following vibrational predissociation of O H - N 2 ( A 2 £ ÷, v ' = 1) are depicted as a function of available energy in Fig. 3. The relative energies of the O H - N 2 intermolecular vibrational levels corresponding to features A, B, and C are also shown. The energy available to OH A Z£+(v ' = 0) fragments has been determined for each O H - N 2 level by subtracting the energy difference between the O H - N 2 level and the OH A 2£+(v' = 1) + N 2 asymptote (AE) from the OH v ' = 1-0 vibrational spacing (2988.6 cm- 1) [26]. AE has been calculated for each O H - N 2 level based on AE=P,(1) +D~-hv, where Pt(1) is the wavenumber for this OH A - X 1-0 transition (35429.14 cm-X), h v is the transition wavenumber required to reach the O H - N 2 level, and D~ is the O H - N 2 ground state binding energy. D~ has been estimated to be 400 cm-1 based on a comparison with N2-HF [16]. The use of this value is supported by the large shift to higher energy (between 150 and 280 cm- 1) of the O H - N 2 (A 2£ +) features relative to OH PI(1); this shift indicates that

c B A

i"..-.

5

OH-N2 2500 o OH A 2,r ÷(v,=0) +N2 (v"=l)

- - I I 2000

1500 8

zo <

1000

7 6

500

5 4 3 2

0

t

- - 0 ott A 2Z ÷(v'=0) + N2(v"~))

Fig. 3. Energy level diagram depicting the OH A 2 £ + ( v ' = 0) rotor levels which may be populated following vibrational predissociation of O H - N z ( A 2 £ +, v' = 1 ) . The vertical axis corresponds to energy available to the OH and N 2 products. O H - N 2 levels are stacked on the left, while OH ( v ' = 0) rotor levels without and with N z vibrational excitation are placed in the center and right, respectively. The available energy for the O H - N 2 levels has been calculated assuming a ground state binding energy of 400 cm -1. (See text.) The bold arrow and bracket designate those OH rotational levels which are plausible product channels following vibrational predissociation from O H - N 2 levels A and B; the gray arrow and bracket denote those levels populated after predissociation from O H - N 2 level C. One should note that a manifold of N 2 rotational levels would be built upon each OH A 2 y + rotor level shown (N 2 rotational constant = 1.998 c m - 1 ).

the ground state is more well bound than the levels observed in the excited state by up to 280 cm -1. This implies that D~(OH-N 2) must be greater than 280 cm-1. Furthermore, we have observed a significant decrease in the fluorescence signal due to O H Ar ( A 2 ~ + ) , by a factor of 10, when 20% N 2 is present (balance Ar), suggesting that D~(OH-N 2) > D~(OH-Ar) = 93 cm i [6]. Lovejoy and Nesbitt [13] have noticed a similar decrease in their A r - H F signal when as little as 6% N 2 has been added; they have also attributed the reduction in signal to a stronger N2-HF ground state binding energy than Ar-HF. O H - N 2 complexes prepared in the level designated A (Fig. 3) have 2744 cm- 1 of energy available

L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9

to be distributed among the OH rotational and translational and N 2 vibrational, rotational, and translational degrees of freedom, while complexes prepared in level B have 2811 cm -1 of excess energy; therefore, the highest energetically accessible channels that can be populated following vibrational predissociation are OH (v' = 0 , n ' = 12), N 2 (u" = 0 ) and OH ( v ' = 0, n ' = 4), N 2 ( u " = 1), where the vibrational frequency for N 2 v " = 1 - 0 is 2329.92 cm-1 [26]. O H - N 2 excited to level C with a relative energy of 2873 cm - l , however, can dissociate to yield fragments in OH (v' = 0, n' = 12), N 2 ( d ' = 0) as well as in levels up to and including OH ( t " = 0, n' = 5), N 2 (v" = 1) since this level is now energetically open. From the dispersed fluorescence spectrum obtained when exciting to O H - N 2 states A and B (corresponding to features A and B), we have observed that the highest energetically open OH rotational level, n' = 12, is populated. Population of this OH level, and those levels with n' >~ 5, signifies that N 2 must be produced without vibrational excitation. Moreover, observation of OH fragments with n' = 12 enables us to place an upper limit on the O H - N 2 ground state binding energy, D'~, of 492 cm - t . We have determined that the majority of the OH fragments ( > 75%) are also created with low n', 0 through 2; this suggests that the O H - N 2 vibrational predissociation event may yield vibrationally excited N 2 ( v " = 1) fragments. Alternatively, the population in low OH n' levels may indicate that excess energy is released as N 2 (u" = 0) rotation (rotor constant is 1.998 cm -1) [26] or the nascent OH and N 2 (l./' = 0) fragments are produced with a large amount of translational energy. By comparison with the vibrational predissociation studies of N z - H F (X 1~+) conducted by Miller and co-workers [16], we can assume that when OH is generated with n' = 0 through 2 the N 2 molecule may come off vibrationally excited. In the case of N2-HF, the small kinetic energy release relative to the rotational energy difference between the two populated HF rotor levels ( j - - - 7 and 12) has led the authors to conclude that the N 2 is vibrationally excited when HF j = 7 is produced [16]. The plausible dissociation pathways from O H N 2 levels A and B are shown with the bold dashed arrow in Fig. 3; the OH fragment levels are bracketed by the bold line.

7

When O H - N 2 is excited to level C, OH rotor levels up to n' = 8 are populated. The N 2 vibrational channel is closed for OH produced in n' = 6 through 8; when OH is produced in these levels, the balance of the energy released upon vibrational predissociation can be deposited in translation of the fragments or rotation of N 2. Further, we note that again when OH A 2~+ n ' = 0 through 5 levels are populated some of the N 2 may be produced with vibrational excitation. The most likely pathways and populated OH product levels for predissociation from level C are depicted in Fig. 3 with the gray dashed line and gray brackets, respectively. The differences between the OH A 2~÷ product rotational distributions for vibrational predissociation from O H - N 2 levels A and B versus C suggest that complexes prepared in these levels access different angular regions of the potential energy surface. From the rotational band contour analysis (Section 3.1), feature C is simulated using a H ~ H transition type versus ~ ~ II (features A and B); this implies that feature C may be due to excitation of an intermolecular bending vibration, while features A and B may arise from intermolecular stretching excitation. Preparation of O H - N 2 in intermolecular stretching or bending levels may result in changes in the OH product rotational distributions, as was the case for O H - A t (A 2]~+, L~'= 1) [6,20,27]. In general, access to various regions of the potential, provided by excitation of different intermolecular vibrations, may result in dissimilar product distributions due to differences in the mapping of the angular motion of the complex on to product rotational channels and/or final state interactions [28].

4. Discussion O H - N 2 complexes excited to intermolecular levels correlating with OH A 2E+(v' = 1) + N: may be removed from these levels by three parallel processes: radiative decay, vibrational predissociation, or quenching. The homogeneous linewidths of the O H - N 2 features may have contributions from all three of these processes; since predissociation is very fast ( ~ 1 ps) relative to radiative decay (700 ns), we can neglect the radiative decay channel. Based on the OH A : E + ( v ' = 1) + N 2 collision studies [5], where

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L.C. Giancarlo, M.I. Lester/ChemicalPhysicsLetters 240 (1995) 1-9

0"01 was determined to be negligibly small compared to o'VET, we will presume that 0"01 is negligible for the complex as well. Complexes prepared in levels derived from OH A 2~+(v' = 1 ) + N 2, therefore, are assumed to undergo vibrational predissociation with near unit quantum efficiency, and the measured lifetime, r, can be ascribed to vibrational predissociation with Zvp = z. O H - N 2 complexes excited to intermolecular levels correlating with OH A 2 X + ( v ' = 0 ) + N 2, however, may only decay by two possible mechanisms which remove population from these levels: radiative decay or a nonadiabatic decay (quenching). We develop a simple kinetic analysis, which makes use of the cross sections, o'VE~ and o-00, obtained from earlier collision work and the measured lifetimes for O H - N 2 complexes prepared with OH ( v ' = 1), to describe this removal of population and corresponding lack of fluorescence from these O H - N 2 (v' = 0) levels. The ratio of the collision cross sections for VET of OH A 2 ~ + ( v ' = 1) with N 2 as compared to quenching of OH A 2E+(v' = 0) by N 2 (O'vET/O'o0 = 36 Az/6.7 ~z) [4,5] is assumed to be proportional to the ratio of the rates for vibrational predissociation (kvp) relative to quenching (kq) in the O H - N 2 complex. While the rates for vibrational predissociation and quenching in the complex may differ from those obtained from collision work, due to the fact that the collisions average over all possible orientations of the collision pair while the complex probes specific configurations, we will assume that the ratio remains unchanged. This relationship suggests that vibrational predissociation of O H - N 2 ( v ' = 1) will occur 5.4 times faster than quenching of O H - N 2 ( v ' = 0). Since complexes prepared in levels A and B predissociate with a lifetime, rvp, of 0.4 ps, the corresponding kvp = 1/rvp is 2.5 ps -1, and, therefore, using this model, kq is assumed to be 0.5 p s - I The total rate of removal of population from O H - N : levels correlating with OH A 2 E ÷ ( v ' = 0) + N2, ktot, is equal to the sum of kq and krad, where krad is the rate of radiative decay, 1/700 ns. Using kq from above, ktot is therefore 0.5 ps -1. The quantum yield for fluorescence from these levels (q0~ = krad//ktot ) is equal to 0, while the corresponding yield for quenching equals 1. Complexes prepared in these O H - N 2 (A2E ÷, v' = 0 ) levels, therefore, do not

fluoresce but rather follow a nonadiabatic decay pathway. The quenching of OH A 2 E + by N 2 may be explained by various models. One such model has been proposed by Crosley and co-workers based on their collision studies of OH A 2E+ with N 2 [1-5]. The quenching cross section was found to be large, to decrease sharply with increasing OH rotational level, and to decrease with increasing temperature. The authors suggest that quenching is controlled by the formation of a transitory collision complex between OH A 2 ~ + and N 2. Further, the establishment of such a collision complex is governed by anisotropic attractive forces in the entrance channel. They assert that mixing of the OH A 2 E + and X 2II states occurs during the lifetime of the collision complex; however, the authors do not speculate on the mechanism for nonadiabatic decay. A theoretical effort into understanding OH A 2~ ÷ + N 2 quenching has recently been undertaken by Paul [29]. In his work, Paul [29] postulates that OH A z~ ÷(v, = 0) is quenched by N 2 due to a harpooning mechanism. He suggests that an electron is first transferred from the N 2 molecule to the OH resulting in OH-; this occurs at the intersection of the OH A 2 ~ + + N2 covalent and OH- 1~ + + N~- ionic surfaces. This ionic surface then crosses with the ground state OH X 2II + N 2 surface on which the OH and N 2 separate. While this mechanism has been successful in calculating the cross sections and temperature dependence for collisions between OH A 2E+ and molecules like H 2 0 and CO, the model predicts o'o0 to be 0 for collisions with N2, which is not in agreement with the experimentally determined value of 6.7 ,~2. Finally, the OH A 2 ~ + + N2 quenching mechanism may also involve reaction to yield either H + N20 [30-32] or NH + NO [33] products. It is known from both experimental [30-33] and theoretical studies [34] that the OH X 21-1+ N 2 potential energy surface correlates with both the H + N20 and NH + NO potentials. Experimental investigations by PatelMisra and Dagdigian [33] have probed the OH X 211 vibrational branching ratios following collisions between NH (X 3~-) ÷ NO (X 211). Moreover, studies by Wittig and co-workers [30-32] have examined reactions of hot H-atoms with N20 to yield NH + NO and OH X 2II + N 2 from the perspective of both

L.C. Giancarlo, M.L Lester / Chemical Physics Letters 240 (1995) 1-9

randomly oriented collisions (full collisions) and the restricted geometry conditions of weakly bound N 2 0 - H I complexes. They have also observed OH A 2•+-X 2I'I chemiluminescence at high H + N 2 0 collision energies under bulk conditions; very little chemiluminescence was detected when N 2 0 - H I complexes were photolyzed at the same energies to induce reaction. Access to the OH A 2 ~ + q_ N2 product channel can only result from a nonadiabatic process [34]. Based on the results of Wittig and co-workers [30-32], we suspect that the reverse reaction, OH A 2 ~ + + N 2 ~ H + N20, may also be occurring under bulk conditions. While it is clear from these experiments that quenching is important in the O H - N 2 complex as well as in full collisions, the origin of this quenching is not fully understood. Further, it is, as yet, unclear whether any of the mechanisms described above for quenching in full collisions will be operative in the O H - N 2 complex. Future work in this laboratory will be directed toward the detection of O H - N 2 in the OH A - X 0 - 0 region via non-fluorescence based techniques.

Acknowledgement This research has been sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences of the Department of Energy. Partial equipment support has been obtained from the National Science Foundation. We thank Dr. Robert W. Randall for his help in the early stages of these experiments. We also acknowledge R. Timothy Bonn for his assistance in the laboratory.

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