Collapse of Vibrational Structure in the Auger Resonant Raman Spectrum of CO by Frequency Detuning

Abstract

Observations made in the C1s ! p ‫ء‬ Auger resonant Raman spectra of carbon monoxide demonstrate that the vibrational structure in such spectra is strongly frequency dependent. In the limit of narrow bandpass photon excitation, the vibrational excitations of the 5s participator transition collapse to one single line within a sub-eV detuning range. The effect is explained in terms of a generalized duration time for the resonance process, in which the lifetime of the core excited state and the inverse of the detuning frequency play equal roles.[S0031-9007 (97)03626-0] PACS numbers: 33.80.Eh, 32.80.HdResonant autoionization of molecules has recently attracted considerable interest from both the experimental and the theoretical points of views. One important ground for this interest is that new synchrotron radiation facilities, notably undulator based sources, have become operative also for gas-phase measurements, and that electron spectrometers, capable of very high resolution even at high kinetic energies, have been developed for use together with synchrotron radiation. Although radiationless resonant inelastic scattering was observed as early as 1980 [1], it was not until 1993 that one could observe the first autoionization spectrum in the vacuum ultraviolet energy range, where the electron lines showed both dispersion with the photon energy and a subnatural line sharpening [2]. This is commonly referred to as the Auger resonant Raman (ARR) effect. Having subnatural resolution in the experiment, both in the excitation and deexcitation, opens up the possibility to study fine effects of detuning the excitation energy from the nominal resonance energy. The pioneering experiment in this context was performed on krypton [3], showing that measured Auger electron line profiles vary strongly with the detuning. This effect, which depends on the distribution of the exciting radiation, i.e., it is specific for the type of monochromator used and of its settings, may in some cases even lead to a doubling of the observed Auger lines. For atoms one has reached a rather detailed understanding of the influence of the photon spectral function on the dispersion law and shape of the x-ray Raman scattering profile, with several theoretical predictions [4][5][6][7] verified by experiment [3,8].It has now become apparent that the dispersion law and the appearance of resonant Raman effect is also strongly influenced by nuclear dynamics, and that spectra of molecules show nontrivial behavior on detuning the frequency [9,10]. Most spectacular is the observation of Skytt et al. [9] that symmetry breaking in the x-ray resonant Raman spectrum of CO 2 is quenched when the excitation energy is detuned from the nominal resonance energy. In the present work, we show that frequency detuning not only is consequential for symmetry selection, but that the vibrational structure in general is distorted or even collapsed upon frequency detuning. This effect is principally different from the type of detuning observed for atoms [3,8].We have measured the ...

Core-level spectroscopy and dynamics of free molecules

“…The vibrational intensity distribution in the Auger spectrum collapses into that of the direct photoionization spectrum for a detuning Ω for which the resonant cross section is still much larger than the direct cross section. As discussed in the original work of Sundin et al [23], the relative intensity follows a smooth, monotonous function of detuning Ω. It should be noted that this is a pure resonant effect.…”

“…In particular, the vibrational fine structure resembles the intensity distribution of the direct photoionization spectrum after a comparatively small detuning value Ω (X-state: Ω = -150 meV; B-state: Ω = -500 meV). This observation is also known as the collapse of vibrational fine structure in the Auger resonant Raman spectrum upon photon energy detuning of the exciting radiation [20,23,24]. It was first observed by Sundin et al [23,24] for negative sub-eV energy detuning relative to the ν' = 0 component of the C1s → π * resonance in CO for the decay into the singly-ionized X 2 Σ + final state of this system.…”

“…This observation is also known as the collapse of vibrational fine structure in the Auger resonant Raman spectrum upon photon energy detuning of the exciting radiation [20,23,24]. It was first observed by Sundin et al [23,24] for negative sub-eV energy detuning relative to the ν' = 0 component of the C1s → π * resonance in CO for the decay into the singly-ionized X 2 Σ + final state of this system. The basis of this effect can decreases drastically already for very small detuning values until it reaches a minimum at ÿ500 meV after which the ratio increases again.…”

“…In this case, when the excitation energy is detuned from the nominal resonant energy, the scattering duration time will be drastically reduced compared to the core-hole lifetime. The nuclear wave packet does not have time to develop in the core-excited state, but the molecule will almost instantaneously decay into the final state [23,24]. The vibrational intensity distribution in the Auger spectrum collapses into that of the direct photoionization spectrum for a detuning Ω for which the resonant cross section is still much larger than the direct cross section.…”

Nuclear motion and symmetry breaking of the B 1s-excited BF3 molecule

“…The lifetime of the core-excited state for light atoms such as B, C, N, O and F is of the order of 10 À15 s, i.e., femtoseconds (fs). Within this short time scale, however, nuclear motion of the molecule proceeds in the core-excited state in competition with the Auger decay [1][2][3][4].…”