Detection Of Nascent Products From The Photolysis Of Acrylonitrile Via Time-resolved Millimeter Wave Spectroscopy

Seifert, Nathan A.

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https://hdl.handle.net/2142/116530 Copy

Description

Title

Detection Of Nascent Products From The Photolysis Of Acrylonitrile Via Time-resolved Millimeter Wave Spectroscopy

Author(s)

Seifert, Nathan A.

Contributor(s)

Prozument, Kirill

Issue Date

2022-06-22

Keyword(s)

Mini-symposium: Spectroscopy meets Chemical Dynamics

Abstract

In 2017, we presented at ISMS a new Time-Resolved Kinetic Chirped-Pulse (TReK-CP) spectrometer.\footnote{Zaleski, D. P.; Prozument, K.; \textit{ISMS} \textbf{2017}, \textit{WH02}.} By coupling a UV photolysis source to a chirped pulse millimeter-wave (mm-wave) spectrometer, we demonstrated the ability to measure kinetic and thermodynamic properties of the photolysis of acrylonitrile (\chem{CH_2CHCN}), including product branching ratios and rotational and vibrational thermalization rates at reasonable time resolution (ca. 10 $\mu$s). However, sensitivity to vibrationally excited states and pre-collisional dynamics was limited, so the observation of truly nascent molecules was still out of reach. Here, we present improvements to the TReK-CP design that enables detection of nascent product molecules from the photolysis of acrylonitrile, with particular focus on the formation of cyanoacetylene (\chem{HC_3N}). Moving to the 260-295 GHz mm-wave band significantly improves sensitivity to small polyatomics, enabling detection of \chem{HC_3N} within 1 $\mu$s after photolysis in a variety of vibrational states. We have also devised a new detection scheme that enables a time resolution of 1 $\mu s$, amongst other improvements. Revisiting the photolysis of acrylonitrile with these improvements has led to surprising observations. We will present evidence that \chem{HC_3N} has different dynamics than the primary photoproduct, \chem{HCN}, which clearly forms rotationally hot and is undetectable until the first collisional event takes place. Meanwhile, cyanoacetylene forms slower, exhibiting low temperature state distributions, a large kinetic isotopic effect, and strong kinetic dependence on the initial temperature of the precursor. This is consistent with the theoretical prediction that the final step, $\chem{CH_2CCN \to HC_3N + H}$, occurs with little excess energy.\footnote{Zaleski, D. P.; Harding, L. B.; Klippenstein, S. J.; Ruscic, B.; Prozument, K. \textit{J. Phys. Chem. Lett.} \textbf{2017}, \textit{8}, 6180.} We will also show that we are, in fact, detecting truly nascent cyanoacetylene, in that the kinetics show a distinct change from first to second order on the collisional timescale of the reactor.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Handle URL

https://hdl.handle.net/2142/116530

DOI

https://doi.org/10.15278/isms.2022.WH07

Copyright and License Information

Copyright 2022 held by the authors

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