Femtosecond time-resolved molecular multiphoton ionization and fragmentation of Na2: experiment and quantum mechanical calculations

Abstract

Abstract. We present a comparison between experimental and theoretical results for pump/probe multiphoton ionizing transitions of the sodium dimer, initiated by femtosecond laser pulses. It is shown that the motion of vibrational wave packets in two electronic states is probed simultaneously and their dynamics is reflected in the total Na~ ion signal which is recorded as a function of the time delay between pump and probe pulse. The time dependent quantum calculations demonstrate that two ionization pathways leading to the same final states of the molecular ion exist: one gives an oscillating contribution to the ion signal, the other yields a constant background. From additional measurements of the Na+-transient photofragmentation spectrum it is deduced that another ionization process leading to different final ionic states exists. The process includes the excitation of a doubly excited bound Rydberg state. This conclusion is supported by the theoretical simulation.

Molecular femtosecond excitation described within the Gaussian wave packet approximation

“…This process has been investigated in great detail. [15][16][17][18][19][20] Figure 1 displays the potential curves for the X 1 ⌺ g ϩ , A 1 ⌺ u ϩ and 2 1 ⌸ g electronic states denoted by ͉0͘,͉1͘ and ͉2͘ in what follows. The ͉1͘←͉0͘ transition induced by a femtosecond pulse of ϭ620 nm peak wavelength and 100 fs temporal width produces a vibrational wave packet in the electronic state ͉1͘.…”

Mapping of quantum phases by two-dimensional vibronic spectroscopy of wave-packet revivals

“…2. At early times (upper panel), the (classical) wave packet oscillates with the classical period T cl of approximately 300 fs [51]. The additional wave-packet phases which stem from the propagation in the anharmonic potential can be identified in writing the |A -state wave function, around the half-revival time, as [33] A (r,t) = e −i π/4 √ 2 [ψ cl (r,t) + e +i π/2 ψ cl (r,t + T cl /2)].…”

Pump/probe direct photoionization from thermally hot samples: the Cs2 molecule

“…For example in the case of the Na 2 pump/probe multiphoton excitation an indirect photoionization pathway which proceeds via a doubly excited Rydberg state has been identified [29,30]. Also, our model does not include any dependence of the transition dipole moments on the bond length and (for the neutral-ionic transition) the photoelectron energy E. We did not include this dependences or tried to model indirect ionization pro- Fig.…”