Ultrafast dissociation and electron excitation in polyatomic organic molecules upon multiple IR photon capture

We suggest a model that allows analysis of the photoprocesses in polyatomic organic molecules upon multiple IR photon capture. The dissociation and electron excitation of these molecules is shown to depend resonantly on the photon energy and intensity. The (CH₂)_n substructures of these molecules serve for IR photons as antennas that accumulate energy to values at which dissociation or electron excitation can be triggered. The model is applied to diphenylalkan molecules as an example.     

Comments on the dissociation of polyatomic molecules by intense 10.6 μm radiation

The discussion of the physical processes involved in the dissociation of polyatomic molecules by intense infrared radiation at a vibrational absorption band leads to an estimate of the required electric field amplitude of (1–3) × 10⁶ V/cm.     

Transition from bloch equations to rate equations for multiple photon excitation of polyatomic molecules

A novel form of phenomenological optical Bloch equations, with density matrix elements depending both on the number of laser quanta in the pumped molecular vibrational mode and on the number of quanta dumped into the thermal bath formed by the remaining vibrational modes, is used to derive rate equations for multiple photon excitation of polyatomic molecules.     

Excitation of polyatomic molecules by radiation

An elementary procedure for calculating quantum mechanically the time-dependent rotation-vibration wave function for a collisionless model of a polyatomic molecule in a monochromatic radiation field is described and applied to some very simple cases. It is concluded that molecules can in principle be strongly and selectively excited at radiation intensities which are too low to produce appreciable excitation in classical calculations. The excitation process is a coherent multi-photon Rabi precession between two discrete levels followed by a transition to a quasi-continuum of vibration-rotation states.     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Potential energy surface of the Rydberg states of the NO molecule in an intense IR radiation field

A method taking into account valence (non-Rydberg) and dissociative configurations has been developed for calculation of potential energy surfaces of the NO molecule in an intense IR radiation field. The resonance rovibronic structure of the Rydberg molecule–laser field quantum system has been analyzed within the steady-state formalism of the radiation collision matrix using multi-channel quantum defect theory. Special conditions for field control of predissociation involving intermediate Rydberg and valence states have been formulated.     

Role of weak transitions in multiphoton excitation of molecules by IR laser radiation

Simultaneous excitation of a considerable part of molecules from many rotational levels of the ground state to higher vibrational states by IR laser radiation can be explained by considering weak transitions in a rotational band structure as it is shown at the example of SF₆ molecule. Very accurate compensation of anharmonicity in relatively wide spectral interval at comparatively low intensity of laser radiation can be explained on this basis. The considered scheme can be applied to the molecules of various symmetry with arbitrary anharmonicity.     

Frequency dependence of the dissociation of polyatomic molecules by radiation

A procedure is described for calculating the dependence of the collisionless dissociation probability of a polyatomic molecule on the frequency of the laser pulse impinging upon it. Sample calculations performed on a simple molecular model show how coherent multiphoton transitions in the vibrational excitation spectrum can be observed in the curve of dissociation probability versus laser frequency.     

On the determination of the reduced rotational operator for polyatomic molecules

Irreducible tensorial sets are used for constructing the reduced rotational Hamiltonian of the molecule, i.e., for obtaining the Hamiltonian with independent rotational, centrifugal distortion, and resonance parameters. General formulas are obtained which allow one to do this, and are valid for molecules of any symmetry. As an illustration of the general results, molecules of T_d symmetry are considered.     

On the excitation of diatomic molecules by intense resonant electromagnetic wave

The vibrational-rotational quasienergy spectra and the excitation probabilities of a diatomic molecule are found, arising under the action of a strong resonance infrared radiation field.     

Laser isotope separation of carbon by multiple IR photon and subsequent UV excitation of CF3I molecules

A new approach to laser isotope separation is considered. It is based on collisionless multiple photon ir laser excitation and subsequent uv laser dissociation of vibrationally excited molecules. TEA CO₂ and excimer XeF, XeCl lasers are used for ir excitation and uv dissociation, respectively. The products of photolysis (C₂F₆) are enriched with¹²C.     

Vibrational combination states of polyatomic molecules investigated by ultrashort two-pulse spectroscopy

A first infrared pulse at frequency ν₁ generates transitions to intermediate states in S₀ and a simultaneous visible pulse at ν₂ raises the excited molecules to a state in S₁. Tuning the frequencies ν₁ and ν₂ over several hundred wave numbers and holding the sum ν₁ + ν₂ constant allows the observation of various combination vibrations. The technique is demonstrated on polyatomic molecules consisting of 42 atoms.     

Experimental selection of molecules for isotope separation by multiple-photon dissociation in an intense IR field

Consideration is given to selectivity estimation methods during polyatomic molecules dissociation by multiple photon absorption. Taking the CH₃NO₂ molecule as an example, the possibilities of selectivity estimations made on the electron-excited dissociation products (luminescence) are shown, as well as on the measurements of the energy absorbed in a strong IR field being transformed into heat. Some advantages of the last method of the selectivity estimation are discussed. The experiments on nitrogen-isotopes separation in the mixture of CH₃ ¹⁴NO₂ and CH₃ ¹⁵NO₂ molecules are carried out when exciting thev ₇ vibration with the isotope shift of about 7 cm⁻¹ and thev ₁₃ vibration with no isotope shift in the linear absorption spectrum. The contribution of secondary chemical reactions to the separation process is discussed.     

Simple models of the density of vibrational states in polyatomic molecules

An analytic approach to calculations of the density of vibrational states in a system consisting of independent anharmonic oscillators was developed. The method worked at all energies from zero point energies to the complete dissociation energies of the selected oscillators.     

Formation of fluorescence bands of polyatomic organic molecules in the gaseous phase upon excitation by electrons

Information on paths in absorption and deactivation of energy gained by molecules in their excitation by electron impact to low-lying singlet states has been obtained from an analysis of changes in the fluorescence spectra of these molecules. It is shown that there is a significant difference in the formation of fluorescence spectra when free molecules are excited by optical radiation and by electrons. It contrast to optical excitation, the interaction of an electron with a molecule is nonselective in character. All electronic states have a chance to be excited, which results in ensembles of emitting molecules with a different store of vibrational energy, and these ensembles each contribute to the fluorescence spectrum. Deceased. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 216–223, March–April, 1998.     

Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca