Possibility of new resonances in transient nonlinear spectroscopy

Vladilen S Letokhov, Veniamin P. Chebotayev

Richard G. Brewer

Although nonlinear spectroscopy evolved slowly in its initial stages, it is now becoming an important new branch of atomic and molecular spectroscopy. A variety of phenomena are being observed for the first time, either in steady state or in transient behavior. In a sense, the methods of microwave and nuclear magnetic resonance spectroscopy are now being reproduced in the optical region, and with the same precision and detail.

Arso Ivanovic

Within this work, electron dynamics inside argon atoms are observed by probing the autoionizing states of argon atoms, so-called window resonances, with both attosecond XUV and timed-delayed few-cycle femtosecond NIR laser pulses, using attosecond transient absorption spectroscopy. The window resonances in the energy region of 25 - 29.3 eV are first excited with the XUV pulse and then dressed by the NIR laser, where the natural decay process of the states is greatly affected by the strong-field of the NIR pulse. Preliminary results of the measurement are presented in this work, with a hypothetical description of some of the effects that are present. A comparison has been made with the experimental results from a paper publishing measurements of autoionizing states with attosecond transient absorption spectroscopy for the first time, where the same resonances were probed. The comparison is only partly possible due to different experimental parameters and conditions, where more pronounced and new effects have been observed in the here performed measurement.

A. H. Zewail

Recently we have reported[1] on the measurements of electronic dephasing (T_2) and radiationless decay (T_1) in molecules using emission spectroscopy as a probe for the detection of the photon echo. This right-angle photon echo utilizes a single mode of a narrow-band (~MHz) cw dye laser and an intracavity electro-optical (EO) element that modulates the frequency of the laser.[2] Applying voltage pulses into the EO crystal switches the laser frequency from ω_0 (the peak frequency of the transition resonance) into another frequency ω. This switching procedure introduced by Brewer et al. for the observation of transients in the forward direction of the laser allows one to observe coherent transients in gases,[1,2] solids[3-5] and molecular beams.[6] Typically, the dispersion of the nonlinear EO crystal (e.g., AD*P) gives a 60 MHz frequency shift within the inhomogeneous resonance of the optical transition (width ~GHz or larger). Therefore, to switch the laser frequency further, one must use high-voltage signals that are usually hard to obtain especially for multiple-pulse experiments. Secondly, there is a limit on the frequency shift which is conditioned by the mode spacing of the cavity. Thirdly, because the laser is switched into a new group of molecules some complications may arise in interpreting the transient spectra especially in large molecules. This is because in large molecules the density of excited states is high and thus the buildup of transients of w molecules (which could be complex) will be superimposed on the decay of ω_0 molecules. Finally, off-resonance effects (e.g., beats) between the molecules at frequency ω and ω_0 cannot be avoided.

Wolfgang Demtröder