Theoretical study of the femtosecond-resolved photoelectron spectrum of the NO molecule

Abstract

The effect of laser fields on the NO interaction potentials is obtained by the calculation of time-resolved photoelectron spectrum ͑TRPES͒ using the time-dependent wave-packet method. The calculation not only shows that the overlap of the pump-probe pulses makes some NO molecular ''invisible'' states visible, but also that the coupling strength and the positions of relevant curves change on increasing the laser intensity. These changed potentials affect their dynamical behavior and influence the shape and position of each peak in TRPES. That the coupling strength of relevant potentials can be changed by the field-matter interaction is consistent with our ab initio calculations. The effect of external field on molecular potential, which is often called field-matter interaction, always becomes an intriguing topic of many dynamical calculations ͓1-6͔. In order to investigate the interaction conveniently, the concept of light-induced potential is introduced and applied to study the photoionization and photodissociation of molecules. For example, using the light-induced potential, Garraway and Suominen ͓7͔ and Zavriyev et al. ͓8͔ studied the process of transferring of a wave packet from one molecular potential to another, and gave the result that a laser-induced crossing becomes an avoided crossing with an energy gap increasing with the laser intensity. In the research of the dissociation dynamics of molecules exposed to intense laser fields, Wunderlich et al. ͓9͔ observed the clear experimental signature of light-induced potentials and pointed out that the avoided crossing moves to a smaller internuclear distance with increasing laser frequency. In this work, we calculated the femtosecond pulse time-resolved photoelectronic spectrum ͑TRPES͒ of NO multiphoton ionization ͑MPI͒, which was obtained by Ludowise et al. ͓10͔ in strong laser fields, and found not only the phenomena of the potential shifting and the coupling strength changing, but also the direct way to detect the ''trapped'' molecules because of the potential crossing.In the experiment of Ludowise et al. ͓10͔, an intense femtosecond pump pulse induces a transition and prepares a coherent superposition of eigenstates, i.e., a wave packet, via the two-photon excitation. The dynamics of the wave packet is then probed with a second, time-delay weak pulse ͑Fig. 1͒. The kinetic-energy spectrum of the ejected electron is subsequently analyzed as a function of the delay. To calculate the MPI TRPES of NO molecule, the five-state quantummechanical calculations are performed. Invoking the BornOppenheimer approximation and neglecting the coupling between the core and photoelectron, the Hamiltonian for the vibrational motion of the molecule can be written aswhere R is the internuclear separation, the reduced mass of the molecule, and the potential matrix U(R,t) can be written explicitly as *Author to whom correspondence should be addressed. Email ad-

Quantum path control on the harmonic emission in the presence of a terahertz field

“…In our calculations, the HHG and the attosecond pulse can be investigated by solving the time-dependent Schrödinger equation (TDSE) [30][31][32][33][34][35][36][37][38] based on single-active electron approximation. In the dipole approximation and the length gauge, the TDSE is given by,…”

Time-dependent wavepacket approach to the influence of intense fields on the population of molecular excited states

“…Since the LMI affects the status of molecules, it must influence the population in each molecular state. In fact, through analyzing the photoelectron spectrum and photodissociation absorption spectrum, one can deduce the population easily [12,17]. In this paper, we will first write out the Hamiltonian of the molecule in intense laser fields.…”

“…Besides above, other methods, such as PIC (particle-in-cell) method [10], the tensor density matrix theory [11], etc., are also used to interpret LMI. Recently, it is noticed that as a powerful tool to study the quantum reactive scattering of chemical reactions, timedependent wavepacket (TDWP) method [12][13][14] has been successfully applied to simulate the quantum dynamics of the ultrafast photoionization and photodissociation of some diatomic molecules in intense laser fields [15][16][17][18]. It is found that when the perturbation theory is not valid in this case, the TDWP method shows its advantages in both the time-saving and the flexibility to deal with LMI.…”

Low-energy structure in the ionization of argon: Comparison of experiment with theory

“…[17][18][19][20][21][22][23][24][25][26][27][28] In the dipole approximation and the length gauge, the TDSE is given by (atomic units are used throughout this paper unless stated otherwise)…”