Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH, CH, and CH Fragments.

Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wide-ranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive.

State-to-State Dynamics in Mode-Selective Polyatomic Reactions.

Chemical reactions are intrinsically quantum mechanical transformations of reactants to products. Recent experimental and theoretical advances have enabled the exploration of reaction dynamics with a quantum state resolution for both reactants and products. To this end, reactions involving more than three atoms are of particular interest, because they exhibit rich dynamics concerning the role of different reactant modes in controlling reactivity and product energy disposal.

State-to-State Dynamics in Mode-Specific Reactions of Cl + CHD(-I, -II, and = 1; |10⟩): Loss of Memory or Not.

Several decades of the study of reaction dynamics culminate in the concept of mode specificity and bond selectivity in polyatomic systems. Until very recently, the main concern of those studies has been total reactivity and little attention has been paid to the mode-specific effects on the more detailed product-state and angular distributions. Conventional wisdom would anticipate that the fine detail should reveal a more pronounced mode dependency.

Stretching-mode specificity in the Cl + CHD(-I, -II, and = 1; |〉) reactions: dependency on the initial |〉 selectivity.

The title reactions were studied at a collisional energy of 5.4 kcal mol in a crossed-beam product-imaging experiment. The dynamics attributes of the dominant ground-state CHD(0) and the accompanied C-D bend-excited CHD(6) products were imaged in reactions with totally 16 ro-vibrationally selected states of the CHD(, |〉) reagents.

Imaging the Mode-Specificity in Cl + CHD(-I, -II, = 1; |⟩ = |10⟩) → CHD(4) + HCl().

We report a crossed-beam imaging experiment on the title reactions at two collisional energies () of 5.3 and 10 kcal mol. Both the integral cross sections relative to the ground-state reactivity and the differential cross sections were measured and compared.

Fermi-phase-induced interference in the reaction between Cl and vibrationally excited CHD.

Mode selectivity is a well-established concept in chemical dynamics. A polyatomic molecule possesses multiple vibrational modes and the mechanical couplings between them can result in complicated anharmonic motions that defy a simple oscillatory description. A prototypical example of this is Fermi-coupled vibration, in which an energy-split eigenstate executes coherent nuclear motion that is comprised of the constituent normal modes with distinctive phases.

Pair-Correlated Imaging of Cl + CHD(, -I, -II = 1, |⟩) → CHD() + HCl().

The title reactions were studied at a collisional energy of 10.0 kcal mol in a crossed-beam, product-imaging experiment. In terms of integral cross sections, all three CH-stretching excited CHD( = 1) reagents promote the reactivity in forming the predominant product pair of (, ) = (0, 0/1) with a prominent mode-propensity of > -I > -II, where denotes the degenerate mode of CH asymmetric stretch and -I and -II are a pair of Fermi-coupled, symmetric-stretch states.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning.

Photodissociation of acetaldehyde (CHCHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond time-resolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.

From Reactive Rainbow to Dynamic Resonance Well.

Scattering resonance is a fascinating phenomenon which manifests as a peak or a dip in an observable as a function of collisional energy (). Its occurrence requires a potential well to support the resonance states. In this regard, reactive resonance is unusual in that it can exist in a reaction with unbound Born-Oppenheimer potential energy surface, on which the quasi-bound states are dynamically trapped-meaning that some energy is temporarily tied to the other degrees of freedom than the reaction coordinate.

Multifaceted Stereoselectivity in Polyatomic Reactions.

Stereoselectivity or stereorequirement refers to the enhancement of chemical reactivity resulting from the preferential alignment/orientation of the colliding reactants. This concept is deeply embodied in the pre-exponential -factor of the Arrhenius rate expression or the entropy effect of thermal kinetics in physical chemistry textbooks. To understand its dynamical consequence and seek for its mechanistic origin, two different approaches of either selecting the rotational states of the reactant or aligning/orienting the reactant in the laboratory have traditionally been taken.

Active stereo-control of the Cl + CH(ν = 1) reaction: a three-dimensional perspective.

The transition state in Cl + CH4 is of Cl-H-C collinear geometry. As the reactant CH4 is vibrationally excited by a linearly polarized infrared (IR) light to the antisymmetric-stretching state of ν3 = 1, all four C-H bonds are collectively excited and any one of the H-atoms can be reactive. Yet, a strong alignment of the excited CH4(ν3 = 1), as evidenced from the striking stereo-specificity in the Cl + CH4 reaction, was clearly revealed in a previous, exploratory study.

Real-time tracking of the entangled pathways in the multichannel photodissociation of acetaldehyde.

The roaming mechanism, an unconventional reaction path, was discovered more than a decade ago in the studies of formaldehyde photodissociation, HCO → H + CO. Since then, observations of roaming have been claimed in numerous photochemical processes. A closer examination of the presented data, however, revealed that evidence for roaming is not always unequivocal, and some of the conclusions could be misleading.

Effects of Stretching Excitations in Cl + CHD( v, v-I, v-II = 1): As the Cl Atom Attacks the Unexcited C-D Bond.

The title reactions were studied at two collisional energies ( E) in a crossed-beam product-imaging experiment. We found that all three initial CH stretching excitations suppress the reactivity toward the abstraction of the unexcited D atom. In terms of vibrational suppression factor, σ/σ, the product channels of CH(0/4) + DCl and CH(1/3) + DCl show opposite mode-specific trends.

Imaging a Resonance-Dominant Polyatomic Reaction: F + CHD → CH(ν = 2) + DF(ν).

The title reaction was studied in a crossed-beam scattering experiment at the collisional energy ( E) ranging from 0.46 to 4.53 kcal mol.

Imaging pair-correlated reaction cross sections in F + CHD(ν = 0, 1) → CHD(ν = 1) + HF(ν).

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1.

Rotational-mode specific effects on the stereo-requirement in the reaction of prealigned-CHD(v = 1; |JK = |10 or |1 ± 1) with the chlorine atom.

Several aspects of the stereo-specific requirement in the title reaction are systematically investigated in a crossed-beam experiment using a time-sliced, velocity-mapped imaging technique. Specifically, we explored (1) the differential steric effect from pre-aligning two different reagent rotational states and (2) the effect from probing different product rotational states. In the reaction with an aligned JK=10 reagent at E = 3.

Imaging spectroscopy of the missing REMPI bands of methyl radicals: Final touches on all vibrational frequencies of the 3p Rydberg states.

(2 + 1) resonance-enhanced multiphoton ionization (REMPI) detection of methyl radicals, in particular that via the intermediate 3p Rydberg states, has shown to be a powerful method and thus enjoyed a wide range of applications. Methyl has six vibrational modes. Among them-including partially and fully deuterated isotopologs-four out of twenty vibrational frequencies in the intermediate 3p states have so far eluded direct spectroscopic determination.

Crossed beam polyatomic reaction dynamics: recent advances and new insights.

Over the past ten years or so, great advances in our understanding of the dynamics of elementary (bimolecular) polyatomic reactions in the gas-phase have occurred. This has been made possible by critical improvements (a) in crossed molecular beam (CMB) instruments with rotating mass spectrometric detection and time-of-flight analysis, especially following the implementation of soft ionization (by tunable low energy electrons or vacuum-ultraviolet synchrotron radiation) for product detection with increased sensitivity and universal detection power, and (b) in REMPI-slice velocity map ion imaging (VMI) detection techniques in pulsed CMB experiments for obtaining product pair-correlated information through high-resolution measurements directly in the center of mass system. The improved universal CMB method is permitting us to identify all primary reaction products, characterize their formation dynamics, and determine the branching ratios (BRs) for multichannel non-adiabatic reactions, such as those of ground state oxygen atoms, O(P), with unsaturated hydrocarbons (alkynes, alkenes, dienes).

Direct mapping of the angle-dependent barrier to reaction for Cl + CHD using polarized scattering data.

The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now.

Imaging characterization of the rapid adiabatic passage in a source-rotatable, crossed-beam scattering experiment.

In order to achieve a more efficient preparation of a specific ro-vibrationally excited reactant state for reactive scattering experiments, we implemented the rapid adiabatic passage (RAP) scheme to our pulsed crossed-beam machine, using a single-mode, continuous-wave mid-infrared laser. The challenge for this source-rotatable apparatus lies in the non-orthogonal geometry between the molecular beam and the laser propagation directions. As such, the velocity spread of the supersonic beam results in a significantly broader Doppler distribution that needs to be activated for RAP to occur than the conventional orthogonal configuration.

Differential steric effects in Cl reactions with aligned CHD(v = 1) by the R(0) and Q(1) transitions. II. Abstracting the unexcited D-atoms.

A complete set of four polarization-dependent differential cross sections in the reactions of Cl + aligned-CHD(v=1,jK)→DCl(v=0)+CHD(v=1) is reported here for two different, rotationally polarized states with j = 1: specifically the jK=10 state prepared via the R(0) excitation and the 1±1 state via Q(1). In stark contrast to the complicated situation of the HCl(v) + CD(v = 0) channel reported in Paper-I, the stereo-requirement of this isotopic channel for both polarized reactants appears quite straightforward and consistent with a direct rebound mechanism. The extent of steric effects is moderate and relatively smaller than the alternative H-atom abstraction channel.

Differential steric effects in Cl reactions with aligned CHD(v = 1) by the R(0) and Q(1) transitions. I. Attacking the excited C-H bond.

When a CHD molecule is pumped to the C-H stretching-excited state by absorbing a linearly polarized infrared (IR) photon via the R(0) branch of the v = 1←0 transition, the rotational angular momentum j of the prepared state jK=10 preferentially lies in a plane perpendicular to the IR polarization axis ε. By way of contrast, when the Q(1) branch is used, the state of jK=1±1 is prepared with j aligned along the direction of ε. Reported here is a detailed study of the title reaction by actively controlling the collision geometries under these two IR-excitation schemes at collision energy E = 8.

Observation of a Reactive Rainbow in F + CH3D → CH2D(v = 0) + HF(v = 3)?

Rainbow structures in the scattering angular distribution play an important role in deepening our understanding about the elastic and rotationally inelastic collisions of atoms/molecules. Reported here is the discovery of a rainbow in a chemical reaction. At Ec = 4.

Control of chemical reactivity by transition-state and beyond.

It has been long established that the transition state for an activated reaction controls the overall reactivity, serving as the bottleneck for reaction flux. However, the role of the transition state in regulating quantum state resolved reactivity has only been addressed more recently, thanks to advances in both experimental and theoretical techniques. In this perspective, we discuss some recent advances in understanding mode-specific reaction dynamics in bimolecular reactions, mainly focusing on the X + HO/CH (X = H, F, Cl, and O(P)) systems, extensively studied in our groups.