Energy Dependence of Photoion Rotational Distributions.

Highly resolved molecular photoionization is a topic of intense current interest due to insights it provides into fundamental scattering processes. Rotationally resolved data provide a window on the angular momentum composition of the photoelectron and the partitioning of angular momentum between the ion and photoelectron. Since the earliest rotational measurements via photoelectron spectroscopy (PES) of H\sb2 using resonance lamp excitation, studies have accelerated as a result of experimental innovations. However, all of these recent developments have been limited to threshold or near-threshold ionization phenomena. In order to bypass this constraint, we have employed dispersed fluorescence measurements. This strategy allows us to exploit the broad tunability of synchrotron radiation because the detection bandwidth is uncoupled from the excitation bandwidth. The isoelectronic target molecules N\sb2 and CO are rotationally cooled by a supersonic expansion, and ionized by synchrotron radiation from a 6-m Plane Grating Monochromator (PGM), at the Louisiana State University Center for Advanced Microstructures and Devices (CAMD). The rotational fluorescence spectra are obtained over the energy range from \rm20\ \le\ h\nu\sb{exc}\ \le\ 220 eV for \rm N\sb2\ 2\sigma\sb{u}\sp{-1} photoionization and from \rm23\ \le\ h\nu\sb{exc}\ \le\ 145 eV for CO 4\sigma\sp{-1} photoionization. The results of \rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+) and \rm CO\sp+(B\sp2\Sigma\sp+) rotational distributions show strikingly different energy dependences. Detailed calculations reveal that the dramatically changing \rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+) rotational distributions arise from Cooper minima of g- and d-waves in the k\sigma\sb{\rm g} photoelectron continuum, while the flatness of \rm CO\sp+(B\sp2\Sigma\sp+) rotational distributions results from a delicate balance between the $4\sigma\ \to\ {\rm k}\sigma\ f$-wave shape resonance and the broad f-wave enhancement in k$\pi$ photoelectron continuum. Cooper minima for \rm N\sb2\ 2\sigma\sb{u}\sp{-1} ionization lead to small $\Delta N$ transitions at lower energies and large $\Delta N$ transitions at higher energies, while the CO $\rm 4\sigma\ \to\ k\sigma$ shape resonance results in larger $\Delta N$ transitions over the whole energy range studied. The present studies of \rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+) and \rm CO\sp+(B\sp2\Sigma\sp+) rotational distributions illustrate effects of Cooper minima, shape resonances, and l mixing in photoelectron continua at energies well beyond the reach of conventional photoelectron spectroscopy. Moreover, these results also demonstrate that rotationally resolved fluorescence is suitable for studies of molecular photoionization dynamics over a broad spectral range

Energy dependence of photoion rotational distributions of N_2 and CO

We present the first measurements of rotational distributions for photoionization over extended energy ranges [0 ≤ E_k ≤ 200 eV for N_2 (2σ^(-1)_u) and of 3 ≤ E_k ≤ 125 eV for CO (4σ^(−1))]. The N_2 and CO results show a strikingly unusual and different energy dependence. 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 photoelectron continuum (kσ_g) in the case of N_2 and from an f-wave shape resonance for 4σ^(−1) photoionization in CO