Nonlinear Fourier-transform spectroscopy revealing wave-packet dynamics of D+2 with multicolor harmonic field

Abstract

We report on a series of studies concerning D2 molecules irradiated by high-harmonic pulses generated from intense femtosecond laser pulses. The kinetic energy (KE) spectrum of dissociated D+ ion fragments with a scanning delay between two replica harmonic pulses exhibits specific characteristics that are completely different from a conventional interferometric autocorrelation signal. We have successfully determined and separated three distinct ionization/dissociation processes by analyzing KE-resolved interferometric fringes by Fourier transform. We call this method for analyzing the KE spectrum of ion fragments "nonlinear Fourier-transform spectroscopy (NFTS)." NFTS provides us molecular information in stationary states because it is intrinsically a frequency domain analysis. Nevertheless, we have resolved the real-time evolution of the vibrational wavepacket of D+ ions with a period of 22 fs by shortening the pulse duration of the fundamental laser pulse to 12 fs and extending the scanning delay range of two harmonic pulses to 150 fs. The probing process of the wavepacket can be described as a model with one-photon absorption of a multicolor harmonic field. We discuss a possible method of reconstructing the phase and magnitude of the wavepacket from the measured delay-KE spectrogram.