Vibrational Feshbach resonances in near threshold HOCO- photodetachment: a theoretical study

Vibrational Feshbach resonances in near threshold HOCO − photodetachment; a theoretical study S. Miyabe, 1, 2 D.J. Haxton, 1 K.V. Lawler, 1 A.E. Orel, 3 C.W. McCurdy, 1, 4 and T. N. Rescigno 1 Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, CA 94720 Department of Chemistry, University of California, Davis, CA 95616 Department of Applied Science, University of California, Davis, CA 95616 Departments of Chemistry and Applied Science, University of California, Davis, CA 95616 The results of a theoretical study of HOCO − photodetachment are presented, with a view toward understanding the origin of two peaks observed by Lu and Continetti (Phys. Rev. Lett. 99, 113005 (2007)) in the photoelectron kinetic energy spectrum very close to threshold. It is shown that the peaks can be attributed to vibrational Feshbach resonances of dipole-bound trans-HOCO − , and not s- and p-wave shape resonances as previously assumed. Fixed-nuclei variational electron-HOCO scattering calculations are used to compute photodetachment cross sections and laboratory-frame photoelectron angular distributions. The calculations show a broad A ′′ (π*)-shape resonance several eV above threshold. I. INTRODUCTION The bimolecular reaction between the hydroxyl radical and carbon monoxide, OH+CO→H+CO 2 , is important in atmospheric and combustion chemistry. As an inter- mediate, the HOCO radical governs the dynamics of this process. For this reason, there has been a considerable amount of experimental and theoretical study aimed at characterizing the molecule [1–3]. The HOCO − anion, which can be formed during the reaction of OH − + CO in a supersonic expansion, has also been the subject of the- oretical [4, 5] and experimental study. Clements et al[6] and Lu et al.[7] showed that the dissociative photode- tachment of the anion provides insight into the potential energy surface of HOCO and the dynamics of the com- bustion reaction. More recently, Lu and Continetti [8] studied the threshold detachment region with 1.60 eV photons and found two sharp peaks in the distribution of photejected electrons at 0.01 eV and 0.09eV, which they interpreted as s- and p-wave shape resonances, respec- tively, based largely on the angular distributions associ- ated with the two features(see Fig. 1). They showed that the latter feature could be used to align the molecular anion by a two-photon detachment process. The assignment of the threshold peaks as shape reso- nances was based on an atomic approximation for pho- todetachment in which the interaction between the pho- toelectron and the neutral molecular core is assumed to be of short range, i.e., to fall off faster than r −2 at large distances. An important factor not considered in this interpretation is the permanent dipole moment of the molecule. For molecules with strong permanent dipole moments the properties of the ejected electron are influ- enced by the dipole field, which is long-range in nature and strongly mixes continuum partial-waves. Because of this mixing, one would not expect for find narrow shape resonances close to threshold. Molecules with sufficiently large dipole moments can bind an electron. Indeed, bind- ing of an electron to the dipole field of a polar molecule is a well known phenomenon. Dipole-bound states are FIG. 1: (Color online) HOCO − photoelectron detachment spectrum at E hν =1.6 eV from Lu and Continetti [8]. (a) Angle-integrated spectrum. Note that peak (III) comes from 2-photon absorption. (b) 2D projection of 3D laboratory- frame photoelectron angular distributions. observed in various experiments and their existence is supported by ab initio calculations [9–11]. Rohr and Lin- der studied the scattering of low energy electrons by HF and HCl and found resonance structure at the vibrational thresholds. The resonances have been ascribed to a tem- porary binding of an electron to the dipole field of the molecule [12, 13]. Dipole-bound states are also observed in the photodetachment spectrum of various anions [14– 18]. Zimmerman et al. were one of the first groups to ob- serve sharp resonances in the photoelectron spectrum of the acetophenone enolate anion [14]. They attributed the resonances to the vibrational levels of the dipole-bound anion. More recently, dipole-bound states have been at- tributed to a series of sharp peaks in the dissociative electron attachment cross sections of DNA bases, RNA base uracil, and the halouracils [19–22]. Similarly, here it will be shown that a dipole-bound state offers a more plausible explanation for the threshold peaks seen in the