Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

Abstract

Articles you may be interested inVibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy J. Chem. Phys. 136, 044304 (2012) Probing cis-trans isomerization in the S 1 state of C 2 H 2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies We report novel experimental strategies that should prove instrumental in extending the vibrational and rotational assignments of the S 1 state of acetylene, C 2 H 2 , in the region of the cis-trans isomerization barrier. At present, the assignments are essentially complete up to ∼500 cm −1 below the barrier. Two difficulties arise when the assignments are continued to higher energies. One is that predissociation into C 2 H + H sets in roughly 1100 cm −1 below the barrier; the resulting quenching of laser-induced fluorescence (LIF) reduces its value for recording spectra in this region. The other difficulty is that tunneling through the barrier causes a staggering in the K-rotational structure of isomerizing vibrational levels. The assignment of these levels requires data for K values up to at least 3. Given the rotational selection rule K ′ − ℓ ′′ = ±1, such data must be obtained via excited vibrational levels of the ground state with ℓ ′′ > 0. In this paper, high resolution H-atom resonance-enhanced multiphoton ionization spectra are demonstrated to contain predissociated bands which are almost invisible in LIF spectra, while preliminary data using a hyperthermal pulsed nozzle show that ℓ ′′ = 2 states can be selectively populated in a jet, giving access to K ′ = 3 states in IR-UV double resonance. C 2015 AIP Publishing LLC. [http://dx

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…C-H bond fission is observed following excitation of C2H2( X , v=0) molecules at wavelengths  < 214.5 nm 14 − corresponding to a photon energy ~600 cm -1 above the bond dissociation energy, D0(H-CCH). 15 Energy conservation arguments dictate that the cofragments formed when exciting at such long wavelengths must be ground (X) state C2H radicals, and measurements of the parent excited state lifetimes, 16 their fragmentation probabilities, 17 the product energy disposal, 18 how these quantities vary with excitation wavelength, 19 and companion ab initio theory 20 all suggest that dissociation occurs via coupling to one or more of the nest of triplet states on a relatively long timescale (long when compared to a typical C-H vibrational period). C-H bond fission following excitation of vibrationally excited C2H2 molecules at longer wavelengths has also been reported (at  ~243.1 21,22 and at 248.3 nm 23 ).…”

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

“…Expansion cooling was always suspected to be rather inefficient in these sources. Nonetheless, by utilizing microwave spectroscopy of cyclopentadienone, high-resolution H atom resonance-enhanced multiphoton ionization of acetylene, and REMPI spectroscopy of xylyl radicals, rotational temperatures of 10–40 K were found upon expansion from a pulsed jet. On the other hand, simulated rotational temperatures of up to 264 K were found for allyl radicals, showing that a substantial amount of internal energy remains in the sample .…”

AC Stark Effect Observed in a Microwave–Millimeter/Submillimeter Wave Double-Resonance Experiment

“…Many experiments utilizing double resonance have been conducted across a wide range of wavelengths, which include radio–microwave, , microwave–microwave, − microwave–millimeter, − microwave–infrared, − microwave–optical, − microwave–ultraviolet, , microwave–X-ray, millimeter–infrared, millimeter–optical, − infrared–infrared, − infrared–optical, infrared–ultraviolet, − infrared–X-ray, optical–optical, − optical–ultraviolet, ultraviolet–ultraviolet, , ultraviolet–X-ray, and phosphorescence–microwave. , Of these previously conducted experiments, it is most relevant to focus on the five previous works reporting microwave–millimeter double resonance. Three of these experiments were conducted by Endo and co-workers, ,, while the other two were conducted by Jäger and co-workers. , These double-resonance experiments were conducted with point-by-point scans, instead of using a chirp or a sweep modulation to achieve broadband coverage.…”