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.     

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.     

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.     

Pulsed optoacoustic spectroscopy for calorimetry wihout a standard

A calorimetric method is described which does not require a reference absorber to deduce absolute yields for heat deposition following pulsed laser irradiation. Pulsed optoacoustic detection is used and signals are calibrated based on the excitation wavelength dependence of the heat yield. The apparatus is used to measure fluorescence yields of several well known dyes in methanol solution. It is also demonstrated on a monolayer of dye adsorbed to sapphire where no reference would be available. This technique was originally developed to measure pulsed temperature rises in stained mitochondria, and to choose an appropriate staining dye for selective thermal denaturation of proteins in cancerous cells by pulsed laser heating.     

Pulsed differential excitation of atoms

It is shown that differential excitation of similar atoms can be achieved by means of a spaced double π/2-pulse sequence in a shorter time than with a single π-pulse, with smaller relaxation degradation.     

Anti-stokes photon replica emission in GaS under two photon excitation

Experimental evidence of light emission due to an anti-stokes photon replica in the extrinsic photoluminescence spectrum of GaS is reported.This non-linear effect has been observed utilizing a high power pulsed ruby laser both for two photon excitation and to provide the electric field for controlling the elementary luminescence event. The experimental result is interpreted on the basis of existing theoretical predictions.     

Optoacoustic detection of multiple photon molecular absorption in a strong IR field

A new method based on the optoacoustic effect has been proposed to measure multiple photon absorption at vibrational molecular transitions in a strong IR laser field. Comparison measurements of the average absorbed energy have been done by this method for molecules with different dissociation limits in a strong CO₂-laser field, the field intensity changed therewith by four orders. For the poly-atomic molecules C₂H₄ and SF₆ having a comparatively low dissociation limit, the absorption increases monotonously as the power density of laser radiation P increases up to the dissociation limit. For three-atom molecules, such as D₂O and OCS, absorption saturation takes place with P ? 10 MW/cm².     

Thermalization in multiphoton excitation of molecules

The thermalization of coherent multiphoton excitation of molecules is examined. An inspection of the level distribution of a driven anharmonic oscillator, which is subject to energy and phase dissipation to a bath, reveals that phase relaxation (dephasing) due to the bath perturbation plays a major role in the thermalization process.     

Pulsed excitation of luminescence in dihydrated oxalic acid

Luminescence has been excited by an a.c. electric field in oxalic acid dihydrate at temperatures 273, 283 and 293 K. The voltage and frequency dependence of the emitted light flux has been studied. The brightness (B) has been observed to increase with voltage (V_rms) and frequency (v). The relation b = b₀ exp(-b/V^{$\text{1}\text{2}$_rms)} describes the voltage dependence of brightness quite accurately at frequencies greater than 500 Hz. The brightness wave forms at different voltages and frequencies of the applied field have also been studied. One primary and one secondary peak have been observed in each half cycle of the applied sinusoidal voltage.     

Multiple photon excitation of polyatomic molecules from the many rotational states by an intense pulse of IR radiation

The investigation of the role of pulse duration in the absorption of CO₂ laser pulses by polyatomic molecules has shown that the rotational relaxation does not contribute significantly to the absorption of intense IR radiation. This proves the existence of purely radiative depletion of all rotational states in monochromatic fields of moderate intensity.     

Multiphoton excitation of complex molecules

A model is proposed for treating complex molecules with extreme vibration energy. The atoms are allowed to move along classical paths independent of their original lattice locations. Inverse Bremsstrahlung is used to estimate the absorption cross section of SF₆. The model is also used to predict vibration to translation energy transfer.     

Multiple photon infrared processes in polyatomic molecules

This paper reviews current understanding of the process of multiple photon excitation and dissociation of polyatomic molecules, whereby in the presence of an intense infrared laser field a molecule may absorb upwards of 30 photons. The application of this process to new photochemistry and in particular laser isotope separation will also be discussed.     

Magnetic excitation of ferromagnetic dimer molecules

The magnetic excitation in ferromagnetic dimer molecules was studied with using the local spin-density approximation. The equation of motion for the atomic magnetic moment was presented. To obtain adiabatic excited states, we implemented a penalty energy technique, which imposes a frozen spin configuration. Calculation conditions of the technique were examined. We applied the method to ferromagnetic dimer molecules, Fe₂, and estimated the magnetic excitation energy. We discussed the magnetic interaction between atoms with mapping the result onto the Heisenberg model.     

Infrared multiphoton excitation of polyatomic molecules

Results of investigations of the process of multiphoton excitation of polyatomic molecules by CO₂-laser radiation are presented. The mechanism of formation of the profiles of IR absorption bands of polyatomic molecules is discussed. New experimental methods of investigation of relaxation processes at high levels of vibrational excitation of molecules in the ground and triplet states are considered. For vapors of polyatomic molecules and their mixtures with foreign gases, the quantitative characteristics of the collisional exchange and the vibrational-energy transfer as well as the rates of intercombinational conversion ⇛ and triplet-triplet transfer are presented and their dependences on the vibrational-excitation level are discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 675–693, September–October, 1998.     

Evolution of ideas in photon detection

In this paper we consider an evolution of ideas in vacuum photodetector developments. Diverse approaches in developments of vacuum photodetectors (classical photomultipliers and hybrid phototubes) for the last half of century are covered. A particular emphasis is made on large area vacuum photodetectors developments. Some other issues concerning WLS and light guide techniques for increasing photodetectors sensitivity are highlighted as well.