Interaction of laser pulses with atoms and molecules and spectroscopic applications

Raman scattering Vibrational levels Pump Stokes Pump anti-Stokes

Raman frequencies in spectrum due to modulation of scattered light by molecular vibrations Inelastic scattering

Electronic-resonance Raman scattering

Characteristic Raman shifts for different bonds A. Fadini and F.-M.Schnepel, Vibrational spectroscopy (Wiley, New York, 1989).

Impulsive excitation of low-frequency modes and pump-probe study of oscillations of molecules and n-particles

Schematic of femtosecond spectroscopy in a pump –probe configuration Delay Pump Probe Detector Sample Temporal response Spectrum Femtosecond pump-probe spectroscopy of n-particles (d~15 nm) N-particle breathing mode oscillations The same principle is applicable for n-particles and molecules

Schematic of the energy levels and optical transitions in CARS

Example: wave interaction in CARS, Phase matching conditions Requirement of phase matching condition k3=k1+k1’-k2; three waves create polarization wave (w3,k3)

Coherent anti-Stokes Raman spectroscopy (CARS) But the CARS signal is limited by limitations on the intensity!!! The object can be destroyed.  - nonlinear susceptibility tensor - wave vector mismatch

Physical values and processes for strong-field laser physics atomic field strength (Hydrogen atom) Intensity required for ionization (Ar) Example: bandwidth requirement for an attosecond pulse: Typical atomic time-scale: Bohr orbit time Typical displacement of an ionized electron in the laser field Corresponding field strength New phenomena: ionization, high harmonic generation (HHG), fragmentation of molecules.

Ionization: Multiphoton and tunnel MECHANISMS Leonid Keldysh, 1964: adiabaticity parameter multiphoton ionization, probability tunnel ionization, probability Atomic system of units

Multiphoton Ionization Kinetic energy of the electron: Ionization probability from perturbation theory: Multiphoton condition (from Keldysh theory): Photoelectric effect (C) Multiphoton Ionization Above Threshold Ionization (ATI) Courtesy of Nathan Hart and Gamze Kaya

Ionization of Argon by femtosecond pulses Ionization of Ar, 200 fs pulses from a Ti:sapphire laser (800 nm). The theoretical ion yields are, from left to right, calculated from Szoke’s model (Perry et al 1988), Perelomov, Popov, Terent’ev, 1966 (PPT) model, Ammosov, Delone Kraynov, 1986 (ADK) theory and strong-field approximation (SFA, Reiss, 1980). S F J Larochelle, A Talebpoury and S L Chin, J. Phys. B: At. Mol. Opt. Phys. 31, 1215 (1998) Multiple ionization of Ar at higher peak intensities of 200 fs pulses from a Ti:sapphire laser (800 nm). S Larochelle, A Talebpoury and S L Chin, J. Phys. B: At. Mol. Opt. Phys (1998)

Ar Dynamics of Ar ionization by femtosecond pulses Arpin et al. PRL 103, (2009)

Electron trajectories after ionization

Cut off for high harmonic generation (HHG) Cut off energy for HHG

Energy of electron returning parent atom

11 th 13 th 15 th 17 th 19 th 21 th (b) 19 th 21 st 17 th 15 th 13 th 11 th 23 d cutoff

Three step model Step 1 Step 2 Step 3 Recombination Electron acceleration in laser field Tunnel ionization XUV P. B. Corkum “Plasma perspective on strong field multi-photon ionization” P. B. Corkum, F. Krausz, “Attosecond Science” S. Haessler et. al., “Attosecond imaging of molecular electronic wavepackets” Courtesy Muhammed Sayrac

Experiments on H 2 + in intense laser fields (simplest molecule) Photodissociation: H nhν H + + H Coulomb explosion: H nhν H + + H + + e - (Pavicic, 2005) At intensities (>10 12 W/cm 2 ) the coupling between 1sσ g and 2pσ u becomes very strong