Alignment of a Molecular Anion via a Shape Resonance in Near-Threshold Photodetachment

Abstract

One-color-two-photon photodetachment of HOCO- at E hnu=1.60 eV accompanied by a measurement of the photoelectron angular distribution (PAD) is used to illustrate a novel approach to the alignment of a molecular anion. A quantitative analysis of the PAD reveals this alignment process is associated with a temporary anion formed by a p-wave shape resonance and the PAD in the two-photon signal is a result of interfering s- and d-partial waves within the atomic approximation. The extracted intensity and phase shift of the partial waves are consistent with the Wigner threshold law for photodetachment.

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

“…The beta parameter, shown in Fig 4b, approaches zero at very low energies. The angular distributions are shown in Fig 5a, and they become more isotropic as energy decreases, which is consistent with the observations of Lu and Continetti [8]. The l = 1 contribution quickly becomes the dominant component as the photoelectron energy increases, leading to a characteristic dipole pattern in the angular distribu- tion, except near the resonance peak, where the l = 1 and l = 2 partial wave contributions become comparable.…”

“…Since the outer electron in dipole-bound HOCO − occupies a region of space far from the neutral HOCO core, it is reasonable to assume the vibrational frequencies of dipolebound HOCO − and trans-HOCO to be very similar. The vibrational energy spacing between the OCO bend levels in trans-HOCO is 0.08 eV [43], which is also the spacing between the threshold photodetachment peaks observed by Lu and Continetti [8].…”

“…For example, a vibrational progression is seen in the photodetachment spectrum of the nitromethane anion [25], which has no known dipole bound states. If the two prominent threshold peaks seen in the experiment Lu and Continetti [8] were the beginning of such a progression, then experiments with higher photon energies should contain those two peaks and perhaps also additional members of the progression. However, such features are not seen in experiments with 3.21 eV photons by Lu et al [7] or in experiments with 4.8 eV photons by Clements et al [6].…”

“…The experiment of Lu and Continetti [8], which used a fixed photon energy of 1.60 eV, could therefore only probe photoelectrons up to a few tenths of an eV produced by single photon absorption. The broad peak in their measured photoelectron spectrum (labeled III in Fig.…”

“…Clements et al [6] and Lu et al [7] showed that the dissociative photodetachment of the anion provides insight into the potential energy surface of HOCO and the dynamics of the combustion 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, respectively, based largely on the angular distributions associated with the two features(see Fig. 1).…”