Marsha I. Lester

Exploring the OH+CO reaction coordinate via infrared spectroscopy of the OH–CO reactant complex

A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO2 reaction. The existence of this linear OH–CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH–CO and OH–OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate.

Experimental determination of the OH-Ar stretching potential

Abstract Interaction potentials between Ar ( 1 S 0 ) and hydroxyl radicals in the A 2 Σ + and X 2 Π 3 2 states are constructed as a function of OH (center-of-mass) to Ar separation distance. The intermolecular potentials are derived from spectral analysis of the OH-Ar fluorescence excitation spectrum observed about the OH A 2 Σ + (ν′=0)-X 2 Π 3 2 (ν″=0) transition. Rotational analysis of features ascribed to an OH-Ar stretching mode in the excited state yields an OH-Ar separation distance of ⩽ 2.9 A at the equilibrium position of the excited state potential and an average OH-Ar van der Waals bond length of 3.6 A at the zero-point level of the ground state.

Optical–optical double resonance of the ICl–Ne complex: Binding energies in the E(0+), A(3Π1), and X(1Σ+) states

Sequential two‐photon excitation of the ICl–Ne complex to ion pair states is achieved via long‐lived intermediate levels in the A state. An entire manifold of van der Waals (vdW) vibrational levels is accessed in the E state. A Birge–Sponer analysis of the vdW stretching motion yields a fundamental vibrational frequency ωe =52.5±0.1 cm−1, anharmonicity ωexe =6.14±0.04 cm−1, and dissociation energy D0=87.6±0.8 cm−1 for ICl–Ne in the E(vE=1) state. Utilizing the spectroscopic shifts of the vdW complexes relative to bare ICl transitions, the vdW binding energies in the ground state, D0=48.2±0.5 cm−1, and all other optically accessed states are determined.

A perturbation theory guide to open‐shell complexes: OH–Ar(X 2Π)

Perturbation theory is used to understand the experimentally observed stimulated emission spectra of OH–Ar(X 2Π). A useful zero‐order Hamiltonian for an open‐shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j⋅s), Renner–Teller coupling (V2), and a Coriolis interaction (J⋅j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open‐shell complexes, such as the large parity splittings in certain vibrational bands and spin–orbit–induced predissociation of the OH–Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O–H–Ar plane, the A’ and A‘ surfaces. The measured splitting is used to infer the magnitude of the difference between the A’ and A‘ potential‐energy surfaces (∼12 cm−1) in the region sampled by the first ex...

Quantum dynamical calculations for the vibrational predissociation of the He-ICl complex: Product rotational distribution

Three‐dimensional quantum close‐coupling calculations are presented for the vibrational predissociation of He–ICl B state complexes containing two quanta of ICl vibrational excitation. The dynamics are evaluated for the lowest quasibound van der Waals levels of He–ICl with total angular momentum J=0 and 1. The vibrational predissociation lifetime and final ICl B(v=1, j ) rotational distribution are calculated using the golden rule approximation. The calculated ICl product rotational distributions are broadly bimodal with maxima at j=7 and 15, as experimentally observed. The computed rotational distributions exhibit pronounced oscillations, which are expected to be suppressed when averaged over the initial angular momentum distribution sampled in the experiment. The theoretical analysis points to the dominant role of final‐state interactions in determining the rotational distribution of the ICl fragments. The zero‐point bending motion of the He–ICl complex and the coupling between the initial and final vib...

Stimulated emission pumping of intermolecular vibrations in OH–Ar(X 2Π)

Stimulated emission spectroscopy has been used to access the intermolecular bending and stretching vibrations supported by the OH(X 2Π)+Ar(1S0) potential‐energy surfaces. Manifolds of OH–Ar bending levels, correlating with the j= (3)/(2) , (5)/(2) , and (7)/(2) rotational levels of OH(2Π3/2), have been observed with zero to three quanta of intermolecular stretch. OH–Ar complexes have also been prepared in intermolecular vibrational levels of the spin–orbit excited state correlating with OH(2Π1/2). The first dissociation limit, producing OH(2Π3/2) v=0, j= (3)/(2) +Ar(1S0) fragments, has been determined to lie between 93 and 103 cm−1 above the zero‐point level. Complexes prepared in metastable levels, detected up to 200 cm−1 beyond the first dissociation limit, undergo predissociation by using OH rotational or spin–orbit excitation to break the OH–Ar intermolecular bond.

Stimulated emission pumping of van der Waals vibrations in the ground electronic state of OHAr

Stimulated emission spectroscopy is used to measure the frequencies of the van der Waals bending and stretching modes in the ground electronic state of the OHAr complex correlating with OH(X 2Π) + Ar(1S0). The results are compared with those predicted using an ab initio potential energy surface. Encouraging agreement between experiment and theory is obtained, particularly for the van der Waals bending modes.

van der Waals vibrational dependence in the vibrational predissociation dynamics of OH–Ar

The OH–Ar vibrational predissociation lifetimes and OH product rotational state distributions are shown to change with van der Waals (vdW) state selection within the manifold of OH–Ar vibrational states correlating with OH A2Σ+(v’=1)+Ar(1S0). Excitations to pure vdW stretching levels result in similar product state distributions, but predissociation lifetimes that vary from 30±8 ps at v’vdW=0 to greater than 150 ps at vvdW=6. Excitations to assigned vdW bend‐stretch combination bands result in product state distributions which differ from those observed after excitation of the pure vdW stretch and those differences are attributed to the form of the bending wave function. Rotational constants and band positions for OH–Ar features in the OH A2Σ+–X2Π3/2 0–0, 1–0, 1–1, 2–1, 1–2, and 2–2 regions are also presented. The spectroscopic analysis reveals details about the radial portion of the intermolecular potential between Ar (1S0) and hydroxyl radicals in the ground X2Π3/2 and excited A2Σ+ states. Vibrational e...

Infrared-driven unimolecular reaction of CH3CHOO Criegee intermediates to OH radical products

Breaking down a Criegee intermediate Ozones damaging role in the upper atmosphere is well known, but ozone is also quite active closer down to where we live. In particular, ozones run-ins with airborne unsaturated hydrocarbons, from natural or anthropogenic sources, produce even more-reactive OH radicals. Liu et al. used vibrational spectroscopy to study how OH emerges from a so-called Criegee intermediate formed when ozone attacks 2-butene. The results suggest that OH production is easier than current theory predicts. Science, this issue p. 1596 Spectroscopy in the laboratory elucidates key steps in ozone’s atmospheric reaction with unsaturated hydrocarbons. Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the troposphere, proceeds through energized Criegee intermediates that undergo unimolecular decay to produce OH radicals. Here, we used infrared (IR) activation of cold CH3CHOO Criegee intermediates to drive hydrogen transfer from the methyl group to the terminal oxygen, followed by dissociation to OH radicals. State-selective excitation of CH3CHOO in the CH stretch overtone region combined with sensitive OH detection revealed the IR spectrum of CH3CHOO, effective barrier height for the critical hydrogen transfer step, and rapid decay dynamics to OH products. Complementary theory provides insights on the IR overtone spectrum, as well as vibrational excitations, structural changes, and energy required to move from the minimum-energy configuration of CH3CHOO to the transition state for the hydrogen transfer reaction.

Vibrational predissociation in OH-Ar

Results are presented showing that the vibrational predissociation dynamics of OH−Ar differs greatly in its ground and excited electronic states. Measurements of the vibrational predissociation lifetime and the nascent internal state distribution of the OH product.(AIP)