Laser-assisted (1+1′)-photon ionization-detected absorption spectrum of the 3pπ 2Π state of HCO and DCO

Abstract

We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the energy of this (040) component and causes an apparent increase in the Renner parameter.

Photodissociation of Acetaldehyde and the Absolute Photoionization Cross Section of HCO

“…In particular, the correspondence of the angular character of the HOMO with an analogous atomic orbital provides useful insight for understanding the present observations. To our knowledge, there has been no measurement of the wavelength dependence of the relative single-photon ionization cross section in the threshold region, although there has been extensive work using double resonance techniques to look at excited-state photoionization spectra. , Given the relatively large energy range of interest (at least 3 eV above threshold), synchrotron-based measurements of the single-photon ionization spectrum would be most beneficial. New measurements of the photoionization spectra of many radicals are now being performed, and HCO would be a prime candidate for future study.…”

Higher Vibrationally Excited Levels of the 3pπ 2Π Rydberg State of HCO

“…Simulations use the value of T o determined by Song and Cool 7 and confirmed in our assisted REMPI scans of the lower vibrational states. 8 χ 22 is calculated, using eq 4, with the mean absolute energies of the Renner-Teller split pairs of Σ states associated with the (010), (030), and (050). Linear regression yields a value of -2.34 cm -1 .…”

“…This process enhances the sensitivity with which we can detect rovibronic band systems. Our original assisted REMPI scans of 3pπ bands as high (030) reveal a number of weak transitions, which, when taken in combination with dominant structure, serve to refine spectroscopic parameters characterizing anharmonic and vibronic coupling, including a Renner parameter that varies with bending amplitude …”

Saddle-node bifurcations in the vibrational spectra of small polyatomic molecules

“…In addition to its practical implications, the very rich resonance structure has attracted a lot of research efforts, both experimental and theoretical, on HCO and especially on its dissociation into H+CO. Comprehensive references can be found in [182][183][184]. Its deuterated isotopomer, DCO, has also been under investigation [115,[184][185][186].…”

“…Comprehensive references can be found in [182][183][184]. Its deuterated isotopomer, DCO, has also been under investigation [115,[184][185][186]. As we will see below, a very shallow potential well (∼ 0.84 eV) characterizes these molecules; resonances are mainly built on excitation of the CO stretching mode.…”