Photoion rotational distributions from near-threshold to deep in the continuum

Abstract

We present the first measurements of ion rotational distributions for photoionization over an extended range ͓0рE K р200 eV for N 2 ͑2 u Ϫ1 ͒ and 3рE K р125 eV for CO ͑4 Ϫ1 ͔͒. The N 2 ion rotational distributions are seen to change dramatically over this energy range, indicating that characteristically molecular behavior of the photoelectron persists far from ionization threshold. In addition, the N 2 and CO results show a strikingly different dependence on energy. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the 2 u →k g continuum in the case of N 2 and from an f -wave shape resonance in the 4 →k channel in CO. Agreement between measured and calculated ion rotational distributions is excellent. The N 2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors.

Vibrational branching ratios in the photoelectron spectra of N2 and CO: interference and diffraction effects

“…At higher energies the RCHF approximation has also been used (CO 43,44,50,52,54,67,[115][116][117][118][119][120] , N 2 3 ). This is in contrast with the frozen core Hartree Fock (FCHF) approximation, which has widely been used in most earlier and some recent works (CO 42,48,58,74,75,106,121 , N 2 22,27,32,58,74,75,106 ).…”

“…However, most of these studies have focused on the valence low-energy region and on the effect of shape resonances in vibrational branching ratios. Only a few of these studies have been performed in the high energy region 58,74,75 . More recent theoretical investigations have addressed the features appearing in the photoelectron spectra at high energy, mostly in core photoionization.…”

“…More recent theoretical investigations have addressed the features appearing in the photoelectron spectra at high energy, mostly in core photoionization. In particular, results for N 2 and CO obtained from these methods have led to the observation of interesting interference phenomena that are often difficult to identify in total photoelectron spectra, especially at high photon energies 4,11,61,64,69,[74][75][76][77][78] . Two of these phenomena, which lie at the heart of quantum mechanics, have been recently observed in vibrationally resolved photoelectron spectra from valence-shell orbitals 78 : (i) two-center coherent electron emission leading to Young's type double-slit interferences 79 and (ii) electron diffraction by the molecule's atomic centers.…”

Vibrational branching ratios in photoionization of CO and N2

“…[1][2][3]5 The excursion in the CO vibrational branching ratio curve at h Ϸ35 eV results from the lϭ3 shape resonance in the 4 →k channel. 2,4 The higher energy regime is featureless because there is no shape resonance at higher energies, and the Cooper minima are relatively weak. 30 For N 2 , the vibrational branching ratios vary from 100 to 200 eV, while at the lower energy the results are comparatively constant.…”

“…In particular, comparisons of isoelectronic systems can illuminate microscopic aspects of photoelectron dynamics in anisotropic molecular potentials. [1][2][3] In studies of N 2 ϩ (B 2 ⌺ u ϩ ) and CO ϩ (B 2 ⌺ ϩ ) photoion rotational distributions, 1,2 we found the energy dependences of the rotational populations to be different, even far from threshold. This behavior is surprising since molecular photoelectron scattering phenomena are not expected to extend far into the continuum, where atomic effects are normally assumed to dominate.…”