Ultrafast molecular reaction dynamics in real-time: progress over a decade

One of the goals of researchers in the field of reaction dynamics is to develop an understanding of the elementary steps involved in a chemical reaction on a molecular level (see e.g. Ref. 1). The century-old Arrhenius rate law, a phenomenological description of the temperature dependence of rates of reactions in bulk, has been used extensively to deduce activation energies and frequency factors. The activated complex theory (also referred to as absolute rate theory or transition-state theory, see e.g. Refs. 2, 3) postulated more than 50 years ago, provides a useful interpretation of the Arrhenius rate parameters in terms of molecular properties. These parameters contain practical information about rates, but they do not express the molecular details of a reaction. At this juncture, two types of questions can be raised--one concerning the effects of the environment on rates in condensed media, and the other, the purely molecular aspects of reactions in the absence of an environment, i.e. in an isolated molecular system. We restrict our attention to the latter case for the purposes of this review

Essays on Economic Conditions and the Living Arrangements of Young Adults

This dissertation contains two essays on the relations between economic conditions and the living arrangements of young adults (22-34 year olds) in the 2000s. In the first chapter, I use data from the American Community Survey and the Current Population Survey for the period 2000-2018 to document recent trends in youth living arrangements and to estimate the impact of state-level economic conditions on individual-level residential outcomes. I find a steep increase in parental coresidence among young adults since 2000. The rise in coresidence is accompanied by declining outflows from the parental home as well as rising inflows into the parental home. Regression results show significant, positive effects of rents on the probability of living with parents relative to all other living arrangements; and significant, negative effects of rising rents on the probability of leaving the parental home. Rents are found to have a larger impact on the living arrangements of non-whites and non-college young adults compared to their respective counterparts. For such youths, rising rents also show a robust, positive association with the probability of returning to the parental home. Overall, rents explain between 9% and 14% of the rise in parental coresidence among young adults over the period 2000-2018. Although the 2000s are also characterized by declining labor market conditions of prime-age workers, changes in prime-age wages are found to explain no more than 5% of the increase in parental coresidence whereas prime-age employment rates show no robust associations with living arrangements. In the second chapter, I take the analysis to the MSA-level and use data on 229 MSAs based on the 2000 Census and the American Community Survey to estimate novel growth models of coresidence. I find significant, contemporaneous effects of growth in earnings and rents, respectively, on growth in parental coresidence among both non-college and college-educated young adults with larger effects on the less-educated. In the long run, however, only the effects of rents are significant such that MSAs which experience higher growth in housing costs during the housing boom of 2000-2006 also experience higher growth in parental coresidence among all young adults over the entire 2000s. In contrast, changes in the employment rates of prime-age workers show no strong associations with living arrangements either contemporaneously or in the long-run. Overall, both chapters of my dissertation imply that rising rents are the main cause of rising coresidence in the 2000s to the extent the latter is an economic phenomenon

Unimolecular reaction rates in solution and in the isolated molecule: Comparison of diphenyl butadiene nonradiative decay in solutions and supersonic jets

The recent study of diphenyl butadiene (DPB) in supersonic jets and in solution by Shepanski et al.(1) and by Courtney and Felming(2), respectively, provides an opportunity to compare the isomerization rates measured in the isolated molecule (jet) with those measured at very low viscosity in solution. These comparisons should shed light on the vibrational energy flows between “optical” and “reactive” modes in the isolated molecule and on the connection between activated, friction dependent, models of barrier crossing in solution,(3-5) and statistical RRK (or RRKM) theories of gas phase unimolecular reactions(6)

Exciton and vibronic effects in the spectroscopy of bianthracene in supersonic beams

Excitation and dispersed fluorescence spectra of 9,9’‐bianthracene in a supersonic expansion are reported. The spectra are anthracene‐like, indicating that the rings are weakly coupled. Exciton effects are considered in the interpretation of the spectra. The torsional potential in S_1 is modeled as a double‐well (Gaussian perturbation on a one‐dimensional harmonic oscillator) with barriers to perpendicularity and planarity of ∼30 and ∼1100 cm^(−1), respectively. The S_0 torsional potential shows negative anharmonicity which is modeled as a quartic perturbation. Anthracenic modes in S_1 and S_0 are also assigned. Finally, measurements of S_1 fluorescence lifetimes up to ∼6000 cm^(−1) excess energy in the excited state show no evidence of charge transfer

Picosecond photofragment spectroscopy. I. Microcanonical state-to-state rates of the reaction NCNO→CN+NO

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state‐to‐state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm^(−1) (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground statepotential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, k_(MC)(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, k_(MC)(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories

Real-time picosecond clocking of the collision complex in a bimolecular reaction: The birth of OH from H+CO_2

Picosecond (and femtosecond) photofragment spectroscopy has recently provided time-resolved, state-to-state dynamics of molecular photofragmentation. The focus of these experiments was on unimolecular reactions, where two main issues are fundamental to the dynamics: the nature of the "half-collision" and the degree to which statistical theories account for the time evolution of product state distributions (PSDs)