Broad vibrational overtone linewidths in the 7νOH band of rotationally selectedNH2OHX. Luo, P. R. Fleming, T. A. Seckel, and T. R. Rizzo Citation: The Journal of Chemical Physics 93, 9194 (1990); doi: 10.1063/1.459210 View online: http://dx.doi.org/10.1063/1.459210 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/93/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Communication: A rotationally resolved (2OH) overtone band in the water dimer (H2O)2 J. Chem. Phys. 141, 111103 (2014); 10.1063/1.4896163 Overtone spectroscopy of the hydroxyl stretch vibration in hydroxylamine (NH2OH) J. Chem. Phys. 102, 675 (1995); 10.1063/1.469179 Intensities in the ν7, ν8, and ν9 bands of CH2NH and the harmonic force field of methyleneimine J. Chem. Phys. 85, 692 (1986); 10.1063/1.451274 Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5νOH) J. Chem. Phys. 84, 1508 (1986); 10.1063/1.450496 The VibrationRotation Bands of OH in the Photographic Infrared J. Chem. Phys. 19, 512 (1951); 10.1063/1.1748273

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Broad vibrational overtone linewidths in the 7VOH band of rotationally selected NH20H

x. Luo, P. R. Fleming, T. A. Seckel, and T. R. Rizzo Department o/Chemistry, University 0/ Rochester, Rochester, New York 14627

(Received 17 September 1990; accepted 8 October 1990)

The relationship between the broad linewidths of light atom stretch overtone transitions and the dynamics of intra­molecular energy flow in overtone excited molecules has been the subject of intense interest and controversy for the last decade. 1-8 To draw unambiguous conclusions from vi­brational overtone linewidths one must eliminate contribu­tions from thermal inhomogeneous broadening. Recent ex­periments have employed both supersonic jet techniques2,6,7,9,10 and double-resonance techniques9,11-13 to simplify vibrational overtone spectra and determine homo­geneous linewidths. We have recently developed an in­frared-optical double-resonance scheme to measure vibra­tional overtone excitation spectra of single rotational states of HOOH. 12-14 Because the vibrational overtone excitation provides the molecules with enough energy to dissociate, we can monitor the overtone absorption by detecting the OH dissociation products. The linewidths of vibrational over­tone transitions measured in this manner can provide infor­mation about the intramolecular energy redistribution and unimolecular dissociation dynamics of the excited molecule. This communication describes our initial infrared-optical double resonance studies of the 7VOH level of hydroxylamine (NH20H). The significance of this report lies in the obser­vation of 14 cm - 1 wide spectral features in the vibrational overtone spectra of rotationally selected NH20H.

Hydroxylamine was prepared by method of Hurd and Brownstein 15 and stored at - 25 ·C. The experiments take place in a room temperature fluorescence cell through which hydroxylamine slowly flows at a pressure of ~50 mTorr. 13

A Nd:YAG pumped optical parametric oscillator (OPO) first excites a particular rotational transition in the OH stretch fundamental band ofNH20H. After a delay of 10 ns, a visible dye laser excites the 7 ..... 1 OH stretch transition and puts molecule above N-O bond dissociation threshold. Ten nanoseconds later a third pulse from a frequency doubled dye laser probes the nascent OH fragments by laser induced fluorescence (LlF) in the A-X band.

Figure 1 (a) shows a vibrational overtone excitation spectrum of the 7VOH level of hydroxylamine using our dou­ble-resonance technique. The OPO excites the PPI (11) tran­sition of OH stretching fundamental at 3625.7 cm - 1,16 pro­ducing molecules with J = 10, K = 0 in the V OH = 1 level. The visible laser is then scanned over the 7 ..... 1 OH stretch transition while the UV probe laser excites the QI (1) transi­tion of OH product fragment. The overtone spectrum con­sists of two 14 cm ~ 1 wide features that we assign to the Qpo( 10) and QRo( 10) transitions ofa near prolate asymmet­ric top. We have measured a series of such spectra in which we vary the intermediate state J and observe the positions of the P and R branch lines change in a manner consistent with their expected 4B(J + 1/2) spacing. In a total of about a 20 double-resonance spectra that we have measured (not all of

which are fully assigned), we observe no lines sharper than ~ 13 cm -I. For contrast, Fig. 1 (b) shows the analogous transitions in the 5vOH + Voo band of hydrogen peroxide obtained using the same double-resonance approach. 14 The similarity in the P to R spacing in the two spectra reflects the similarity of the B rotational constant of the two molecules. Because the transitions in Figs. 1 (a) and 1 (b) produce excit­ed molecules at similar energies above their respective disso­ciation thresholds, their comparison provides insight into the origin of the broad NH20H vibrational overtone linewidths. The three most likely sources for this broadening are (1) spectral overlap from transitions with different ini­tial states (i.e., inhomogeneous structure); (2) lifetime broadening due to fast unimolecular dissociation; and (3) eigenstate structure resulting from vibrational state mixing at the 7VOH level of hydroxylamine. We consider each of these possibilities below and conclude that vibrational state mixing at the 7VOH level is the most feasible explanation for the 14 cm - 1 linewidth of the spectral features.

18000 18050 (b) HOOH, 5voH+voo

0(>0(10)

12600 12650

Wavenumber (em-I)

FIG. 1. (a) Double-resonance vibrational overtone excitation spectrum of the 7VOH level of hydroxylamine monitoring the Q, (I) transition of the OH product fragment (see text for details). Excitation ofNH20H molecules via the QP,,( 10) transition in this spectrum prepares them with J = 9, K = 0 and -128 cm- 1 of excess energy. (b) Double-resonance vibrational over­tone excitation spectrum of the 5vOH + Voo level of hydrogen peroxide (Ref. 14) monitoring the Q, (1) transition of the OH product fragment. Excitation of HOOH molecules via the QP,,( 10) transition in this spectrum prepares them with J = 9, K = 0 and -129 cm - , of excess energy.

9194 J. Chem. Phys. 93 (12), 15 December 1990 0021-9606/90/249194-03$03.00 @ 1990 American Institute of Physics

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Letters to the Editor 9195

High resolution infrared spectroscopy of the hydroxyla­mine OH stretch fundamental reveals a hybrid band of a near prolate asymmetric top. 16 Table I lists the ground state rotational constants and asymmetry parameter along with those of hydrogen peroxide. Because B ofNH20H is similar to that of HOOH, the degree of selectivity achieved by our -0.1 cm - I OPO should be essentially the same for the two molecules. Moreover, because we excite perpendicular tran­sitions in the infrared step, there is no possibility of overlap between transitions with the same J but different K, since they are spaced by 2 (A - B) or - 11 cm - I. Furthermore, the NH20H fundamental band lacks the overlapping vibra­tional bands and torsional splittings that complicate the HOOH spectrum. 17 Taken together, the spectroscopic evi­dence suggests that double-resonance vibrational overtone spectra of NH20H should be of equal or higher spectral pu­rity than those of HOOH. 13

•14 We conclude that thermal

inhomogeneous structure (i.e., transitions from different in­itial states) cannot be the major source of the 14cm- 1 vibra­tional overtone linewidths of NH20H.

Because the 7VOH level of NH20H is above the N-O dissociation threshold, the finite lifetime of the molecule, t d ,

will broaden the vibrational overtone transitions by an amount ail = (21TCtd ) -I. For this to be the primary source of the 14 cm -Ilinewidths, the excited molecules would have to dissociate in 380 fs. Although this is too fast to measure directly with our present apparatus, comparison of the NH20H overtone transition linewidths with those of HOOH at a similar excess energy above its dissociation threshold allows us to estimate the degree of lifetime broad­ening caused by the NH20H dissociation. We determine the excess energy of the excited NH20H molecules by measur­ing the quantum state distribution of the OH dissociation products (via LIF) and assuming that all states that con­serve energy and angular momentum will be populated. 1M

Upon dissociation of NHPH molecules with J = 9, K = 0, and VOH = 7, 73% of the OH fragments are produced in the N = 1 level and 27% in the N = 2 level. 19 A phase space theory calculation predicts that this observed OH product state distribution requires 128 cm - I of excess energy. 20 We chose the 5vOH + Voo vibrational overtone band of HOOH [Fig. 1 (b)] for comparison with the NH20H spectrum be­cause peroxide molecules with J = 9 and K = 0 contain roughly the same excess energy above the dissociation threshold. 14 Statistical calculations of the unimolecular de­cay rate predict the hydroxylamine molecules excited via the

TABLE I. Ground state rotational constants (in cm - ') and asymmetry parameter of NH20H and HOOH.

K

"From Tsunekawa (Ref. 23). "From Olson et al. (Ref. 24).

6.326

0.840 938 0.834666

0.837802

- 0.997 7

HOOH"

10.06944

0.873702 0.837848

0.855775

- 0.992 2

transitions in Fig. 1 (a) will dissociate an order of magnitude slower than hydrogen peroxides excited via the transitions in Fig. 1 (b). 21 The difference in the statistically calculated rate results primarily from a 40 times higher density of states for hydroxylamine than for HOOH at the same excess energy. The lifetime broadening contribution to the NH20H vibra­tional overtone linewidths should therefore be less than that ofthe 5vOH + Voo transitions ofHOOH. Although this con­clusion is based on the assumption of statistical unimolecu­lar dissociation rates, the relative rates NH20H and HOOH would have to differ by two orders from the statistical pre­diction for our conclusion to be affected.

Having ruled against unresolved spectral congestion and dissociative lifetime broadening as major contributors to the 14 cm -I vibrational overtone linewidths ofNH20H, we suggest that the broad spectral widths arise from vibrational state mixing. The zeroth-order bright state mixes with near­ly isoenergetic dark states, resulting in a clump of mixed molecular eigenstates that fill in a Lorentzian envelope. If these eigenstates were sufficiently well spaced such that they did not overlap within their naturallinewidth, one could in principle resolve them; however, the spectral width of each eigenstate imposed by the finite lifetime of the dissociating molecule makes this possibility unlikely. The 14 cm - I linewidths of the transitions in Fig. 1 (a) therefore determine the rate at which energy in the OH stretch vibration would couple to the rest of the molecule if the entire feature were coherently excited. This type of behavior is not unique to NH20H. We have observed - 15 cm - I vibrational overtone linewidths at the 7VOH level of HOOH which is -4400 cm - I above the dissociation threshold. 22 Although we have no way of directly determining the relative magnitudes of vibrational state mixing and lifetime broadening at this high excess energy, our observation of 14 cm- I vibrationalover­tone linewidths at the 7VOH level of NH20H suggests that the widths of the 7VOH levels of HOOH might also arise largely from vibrational state mixing.

These broad vibrational overtone linewidths are par­ticularly intriguing in light of the recent experiments of Page, Shen, and Lee in which they observe a - 10 cm - I vibrational overtone linewidth at the 3vCH level of jet-cooled benzene.9 The trend in their spectra as VCH increases from 1 to 3 suggests that the average coupling matrix elements, w, decrease, compensating for the large increase in density of states and keeping the homogeneous linewidth relatively narrow. They predict that the CH overtone linewidths should not exceed 10 cm - I for VCH > 3. Our observation of 14cm- l linewidths at the 7VOH level ofNH20H implies that larger molecules with comparable couplings will have even greater homogeneous linewidths at similar levels of excita­tion. Weare cautious about generalizing our results, how­ever, since the presence of specific resonances in NH20H could result in unusually large coupling matrix elements. Theoretical studies of the vibrational coupling in NH20H as well as spectroscopic studies of other vibrational levels can help determine if this is the case.

We gratefully acknowledge the support of this work by the Office of Basic Energy Sciences of the Department of Energy. We also thank the donors of the Petroleum Re-

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9196 Letters to the Editor

search Fund, administered by the American Chemical So­ciety, for partial support of this research.

IK. V. Reddy, D. F. Heller, and M. J. Berry, J. Chern. Phys. 76, 2814 ( 1982).

2G. A. West, R. P. Mariella, J. A Pete, W. B. Hammond, and D. F. Heller, J. Chern. Phys. 75, 2006 (1981).

·'K. von Puttkamer, H. R. Diibal, and M. Quack, Faraday Discuss. Chern. Soc. 75,197 (1983).

4E. L. Sibert, W. P. Reinhardt, and J. T. Hynes, J. Chern. Phys. 81, 1115 (1984).

'M. C. Chuang and R. N. Zare, 1. Chern. Phys. 82, 4791 (1985). "E. S. McGinley and F. F. Crim, J. Chern. Phys. 85, 5741 (1986). 7L. J. Butler, T. M. Ticich, M. D. Likar, and F. F. Crim, J. Chern. Phys. 85, 2231 (1986).

"A. Amerein, H. Hollenstein, M. Quack, R. Zenobi, J. Segall, and R. N. Zare, J. Chern. Phys. 90,3984 (1989).

"R. H. Page, Y. R. Shen, and Y. T. Lee. J. Chern. Phys. 88, 4621 (1988). lOB. R. Foy, M. P. Casassa, 1. C. Stephenson, and D. S. King, J. Chern.

Phys. 92, 2782 (1990). "s. L. Coy and K. K. Lehmann, 1. Chern. Phys. 84, 5239 (1986). 12X. Luo, P. T. Rieger, D. S. Perry, and T. R. Rizzo, J. Chern. Phys. 89, 4448

(1988).

IJX. Luo and T. R. Rizzo, J. Chern. Phys. (in press). 14x. Luo and T. R. Rizzo, J. Chern. Phys. (in press). "C. D. W. Hurd and H. J. Brownstein, J. Am. Chern. Soc. 47, 67 (1925). 16M. E. Coles, A. J. Merer, and R. F. Curl, 1. Mol. Spec. 103, 300 (1984). '7Both infrared and microwave studies of NH20H find only the trans iso-

mer present, and ab initio estimates of7 kcal!mol for the energy difference between the cis and trans isomers support this observation.

I "Our experience with hydrogen peroxide suggests that phase space theory almost always correctly predicts that there will be some population in all levels that satisfy the energy and angular momentum constraints.

'"The N quantum number of OH is the total angular momentum minus electronic spin.

2°This determines a value of 21 620 ± 20 cm - I for the N-O bond energy, which is 186 cm - I higher than the thermodynamically determined value ofR. A. Back and 1. Betts, Can. J. Chern. 43, 2157 (1965).

21The statistical expression for the unimolecular dissocation rate is propor­tional to the ratio of the number of open channels (states) at the transition state to the density of states of the parent molecule. While various statisti­cal theories count the number of open channels differently, these differ­ences will have little effect on the relative rates and will not change our conclusion. We use phase space theory here to count the number of open product channels.

22x. Luoand T. R. Rizzo (unpublished). 2 .. S. Tsunekawa, 1. Phys. Soc. Jpn. 33,167 (1972). 24W. B. Olson, R. H. Hunt, B. W. Young, A. G. Maki, and J. W. Brault, J.

Mol. Spec. 127, 12 (1988).

J. Chem. Phys., Vol. 93, No. 12,15 December 1990

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