Browsing by Author "Chin, S. L."

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Direct observation of super-excited states in methane created by a femtosecond intense laser field
(2008-11-28)
Azarm, A.
;
Xu, H. L.
;
Kamali, Y.
;
Bernhardt, J.
;
Song, D.
We report, for the first time, a direct observation of the super-excited states of CH(4) in femtosecond intense laser fields using a pump (800 nm)-probe (1338 nm) technique. An unambiguous depletion of the CH (A (2) Delta -> X (2)Pi) fluorescence signal as a function of the delay time is attributed to the de-excitation of the super-excited states by the probe laser pulse. The lifetime of the super-excited state is measured to be about 160 fs.
Experimental and theoretical investigation of high-power laser ionization and dissociation of methane
(2007-09-27)
Sharifi, A.
;
Kong, F.
;
Chin, S. L.
;
Mineo, H.
;
Dyakov, Y.
The main purpose of this paper is to report the high-power laser ionization-dissociation of CH4 at various femtosecond (fs) laser intensities (from 1 x 10 14 W/cm(2) to 2 x 1015 W/cm(2)) with a laser pulse duration of 48 fs. The generalized molecular Keldysh theory has been applied to calculate the ionization yields for CH4+ and CH4++. Outside the influence of the A intense laser, we propose to calculate the mass spectra due to the decomposition of CH4+ and CH4++, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The agreement between the experimental mass spectra and calculated mass spectra seems to be reasonable.
Highly Multiphoton Excitation of Molecule by Intense Laser Field
(2009)
Teranishi, Y.
;
Hayashi, M.
;
Song, D.
;
Azarm, A.
;
Kong, F.
Multiphoton excitation of molecule by intense laser field is studied theoretically to explain the recent experiments of neutral fragmentation Neutral fragments are created from highly excited parent molecule Excitation mechanism is discussed with the use of Floqet theory.
Highly multiphoton molecular excitation by an intense laser pulse
(2008)
Teranishi, Y.
;
Hayashi, M.
;
Kong, F.
;
Chin, S. L.
;
Chao, S. D.
Multiphoton excitation of molecules by an intense laser field is treated. When the first excitation energy is much larger than both the photon energy and the transition energy between excited states, excited states are strongly coupled with each other by the photon, even though coupling with the ground state is weak. In such a case, a kind of collective excitation takes places, in which the excitation probability of every excited state is almost linear as functions of intensity on a log-log plot, and the slope is independent of the final state. By taking methane and formaldehyde as examples we numerically study this process.
Neutral dissociation of hydrogen molecules in a strong laser field through superexcited states
(2011-04-28)
Azarm, A.
;
Song, D.
;
Liu, K.
;
Hosseini, S.
;
Teranishi, Y.
Fragmentation of hydrogen molecules in a strong laser field is studied experimentally and theoretically. Balmer lines (n -> 2) from hydrogen atoms are observed up to n = 14. The emission intensity of the two strongest Balmer lines versus laser intensity in a log-log plot gives a slope of more than 11. This indicates that some high-lying states in the continuum are excited resulting in neutral dissociation. Adapting Rydberg state approximation, the potential energy curves of superexcited states of hydrogen are plotted. Semi-empirical calculation confirms dissociation through these superexcited states. The upper limit of the lifetime of the superexcited states is obtained by an ultrafast pump and probe experiment by monitoring one of the fluorescence lines.
Neutral dissociation of simple molecules in strong laser field
(2011)
Azarm, A.
;
Song, D.
;
Teranishi, Y.
;
Hosseini, S.
;
Lin, S. H.
we report neutral dissociation of simple molecules in strong laser field, experimentally. Moreover, theoretical calculations of potential energy curves justify neutral dissociation through super excited states in all of the investigated gases. (C) 2010 Optical society of America
Population trapping and rotational revival of N(2) molecules during filamentation of a femtosecond laser pulse in air
(2010-12-14)
Azarm, A.
;
Ramakrishna, S.
;
Talebpour, A.
;
Hosseini, S.
;
Teranishi, Y.
We study the fluorescence emitted from filaments in air using a pump-probe scheme with a femtosecond Ti-sapphire laser. The fluorescence intensities from the first negative band (B(2)Sigma(+)(u) -> X(2)Sigma(+)(g)) and the second positive band (C(3)Pi(u) -> B(3)Pi(g)) show enhancement and change periodically as a function of the pump-probe time delay. We attribute this phenomenon to the universal yet probably forgotten phenomenon of population trapping of nitrogen molecules in highly excited states together with field-induced alignment of nitrogen molecules followed by revivals of the rotational wavepackets. Theoretical calculation of the alignment dynamics of nitrogen molecules is consistent with the experimental data.
Population trapping and rotational revival of N-2 molecules during filamentation of a femtosecond laser pulse in air
(2010-12-14)
Azarm, A.
;
Ramakrishna, S.
;
Talebpour, A.
;
Hosseini, S.
;
Teranishi, Y.
We study the fluorescence emitted from filaments in air using a pump-probe scheme with a femtosecond Ti-sapphire laser. The fluorescence intensities from the first negative band (B-2 Sigma(+)(u) -> X-2 Sigma(+)(g)) and the second positive band (C-3 Pi(u) -> B-3 Pi(g)) show enhancement and change periodically as a function of the pump-probe time delay. We attribute this phenomenon to the universal yet probably forgotten phenomenon of population trapping of nitrogen molecules in highly excited states together with field-induced alignment of nitrogen molecules followed by revivals of the rotational wavepackets. Theoretical calculation of the alignment dynamics of nitrogen molecules is consistent with the experimental data.
Studying the neutral dissociation of O-2 and CH4 Molecules in strong laser field
(2009)
Azarm, A.
;
Kamali, Y.
;
Bernhardt, J.
;
Xu, H. L.
;
Song, D.
We report neutral dissociation of oxygen and methane in femtosecond laser field through superexcited states. Moreover, the lifetime of the superexcited state is measured by pump and probe technique to be about few hundred femtosecond. (C)2009 Optical Society of America