Photoelectron-recoil-induced rotational excitation of theBΣu+2state inN2

Abstract

In the photoelectron spectrum of N 2 the apparent ionization energy to form the B 2 ⌺ u + state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.When a photoelectron is ejected from an atom, molecule, or solid the remaining ion has a recoil momentum that is equal and opposite to that of the electron. Although the first discussions of this effect ͓1͔ were limited to translational recoil, it was recognized by Domcke and Cederbaum ͓2͔ that the recoil effect could lead to internal excitation of the ion. However, this prediction remained unverified until recent observations in core-electron photoelectron spectra of recoil excitation of vibrations in molecules ͓3,4͔ and phonons in solids ͓5͔. These experiments show that the recoil-induced internal excitation is quantitatively in accord with a model based on emission of the electron from a localized atom.Although a model based on emission from a localized atom may be appropriate for core ionization, it is not apparent that such a model is appropriate for valence ionization, where the electrons are delocalized. This question has been recently addressed by Takata et al. ͓6͔ who showed that at a photon energy of 8 keV there is a shift in the apparent position of the Fermi edge of aluminum that is consistent with the recoil being taken up by a single atom.The investigations mentioned above have been concerned with vibrational excitation. Here we consider recoil-induced rotational excitation during valence photoionization of N 2 . Thus we extend the previous investigations by considering a different type of internal excitation and by considering valence excitation in a distinctly different system ͑a small molecule rather than a solid͒. Specifically we investigate rotational excitation during photoionization to produce the B 2 ⌺ u + state of N 2 + . Using both photoelectron and fluorescence spectroscopy we show that there is recoil-induced rotational excitation in quantitative accord with a model based on emis-sion of the electron from a localized atom. Moreover, we note that this effect is observable even within 100 eV of threshold. Thus it becomes apparent that significant recoilinduced internal excitation is widespread in terms of both the physical system ͑molecule or solid͒ and the energy range.It has been previously noted that the distribution of rotational states produced during photoionization to form the B 2 ⌺ u + state of N 2 + depends on the photon energy ͓7,8͔. The distribution shifts to higher values of the rotational quan...

Atomic Auger Doppler effects upon emission of fast photoelectrons

“…Another manifestation of vibrational recoil was observed in Ne dimers, and it was shown that a significant modification of the nuclear dynamics accompanying interatomic Coulomb decay is induced by the recoil momentum imparted on the nuclei by fast photo-and Auger electrons 12 . Some years later rotational recoil was also observed 13 . Using hard X-rays of several keV in energy, the recoil effect was also observed in the valence 14 and core ionization 15 of solids.…”

Site-selective photoemission from delocalized valence shells induced by molecular rotation

“…However, this first impression is not correct, as one can see from the valence photoionization amplitude c 5s ðe Á rÞ j jc k i¼ P n e ikÁR n b n ðkÞ , which is the sum of the ionization amplitudes for the hydrogen b H (k), and for the chlorine b Cl (k) atoms. Here the factor e ikÁR n is responsible for the momentum and the angular momentum transfer from the electron to the molecule [13][14][15][16][17][18][19][20][21][22][23][24] . The radius vector (lever) R n between the nth atom and the centre of gravity of the molecule is large for the light hydrogen atom, but it is very small for the heavy chlorine atom (R Cl /R H ¼ m H /m Cl E0.028).…”

Core-level spectroscopy and dynamics of free molecules

“…Experimental work on core ionization in CH 4 and CF 4 (see Refs. [68,69] and references therein) and valence ionization in N 2 [70] has shown that the recoil of the atom where the vacancy is created causes non-Franck-Condon effects well above the ionization threshold, and induces the excitation of vibrational or rotational modes otherwise inactive.…”

“…A different approach was used by the same authors in Ref. [70], where the recoil effect has been observed in valence spectra, and in which the momentum transfer leads to the excitation of rotational rather than vibrational modes. The system under investigation is N 2 , and valence photoelectron spectra are reported as a function of photoelectron kinetic energy and compared with previous fluorescence data.…”

“…28, taken from Ref. [70], the rotational heating in the singly-ionized B valence final state of N 2 is shown as a function of photoelectron kinetic energy due to the increased population of rotational levels caused by the recoil.…”