IR Multiphoton Absorption Research Articles

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF6 molecules as tiny probe thermometers is described, and results are presented, for large (CO2)N van der Waals clusters (with N ≥ 102) in a cluster beam. The SF6 molecules captured by (CO2)N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF6 in the incoming beam. As a result, the nanoparticle temperature in the (CO2)N cluster beam is estimated as Tcl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO2 laser are performed to determine the velocity (kinetic energy) of SF6 molecules sublimating from clusters, and the cluster temperature is found to be Tcl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.

Vibration Energy Accumulation and Redistribution in Organic Molecules Irradiated by Infrared Photons

A theoretical approach to the dissociation and low-energy electronic excitation of polyatomic organic molecules with donor and acceptor substructures is suggested. The donor hydrocarbon molecular substructures can serve as antennas for low-energy infrared (IR)-photon absorption, which coherently induce collective vibrational excitations (excimols). Due to dipole-dipole interactions, the accumulated energy can transit to the molecular acceptors: dipole-type trap-bonds or molecular parts with π-electron orbits. The analytical expressions for the probability functions of molecular fragmentation and electronic excitation induced by IR-multiphoton absorption are derived. The vibrational energy accumulation and redistribution in the molecules of diphenylalkanes irradiated by infrared photons are considered from the presented point of view.

Spectral and energetic characteristics of passing vibrationally excited SF6 molecules through a cooled multichannel plate

We study the spectral and energetic characteristics of SF6 molecules excited in a pulsed molecular beam by intense IR laser radiation into high-lying vibrational states (with the energy Ev ≥ 0.3–2.0 eV) that pass through a multichannel metal plate that is cooled to Ts ≅ 80–85 K and is inclined to the beam axis. From comparison of the measured characteristics with the spectral and energetic characteristics of the IR multiphoton absorption of these molecules, we find that they correlate rather well with each other. The results obtained allow us to conclude that, due to different probabilities of passing vibrationally excited and unexcited molecules through the multichannel plate, it is possible to separate molecules in the beam with respect to their isotopic (component) composition.

What can the activation process model give the adiabatic and non-adiabatic approximation?

In the framework of the adiabatic approach: the notion of a quantum subsystem for a separate molecule has been determined; the mechanism of transition from the previous quantum subsystem to the subsequent one has been offered; the calculation of average energy of high-energy rotational spectral components has been carried out from first principles for the diatomic molecule; the average energy of the translation of Hg and H atoms, participating in the HgH molecule elementary dissociation process, has been estimated; the overall system of equations for the elementary dissociation process of the molecule has been received; the notion of a quantum subsystem for a protein molecule has been defined; on the basis of this definition the kinetics of folding and insertion for the β-barrel outer membrane protein A (OmpA) of Escherichia coli into dioleoylphosphatidylcholine (DOPC) bilayers has been simulated numerically; the possible adequacy of the model to dynamic properties of the protein mobility has been shown. For the non-adiabatic approach: the quantum subsystem notion has been given; the phenomena of IR multi-photon absorption and photodissociation for molecules in the intensive field of laser radiation have been explained; the transcendental equation for calculation of a number of identical IR quanta absorbed by the molecule under its dissociation has been derived from first principles; the estimation of this parameter for the SF 6 molecule has been carried out, and good coincidence with experimental results has been shown. In accordance with the model proposed, the concept of molecular structure has been outlined.

Multiphoton Near‐Infrared Femtosecond Laser Pulse‐Induced DNA Damage With and Without the Photosensitizer Proflavine

Abstract— The excitation of pBR322 supercoiled plasmid DNA with intense near‐IR 810 nm fs laser pulses by a simultaneous multiphoton absorption mechanism results in single‐strand breaks after treatment of the irradiated samples with Micrococcus luteus UV endonuclease. This enzyme cleaves DNA strands at sites of cyclobutane dimers that are formed by the simultaneous absorption of three (or more) 810 nm IR photons (pulse width ∼140 fs, 76 MHz pulse repetition rate, average power output focused through 10×microscope objective is ∼1.2 MW/cm2). Direct single‐strand breaks (without treatment with M. luteus) were not observed under these conditions. However, in the presence of 6 μM of the intercalator proflavine (PF), both direct single‐and double‐strand breaks are observed under conditions where substantial fractions of undamaged supercoiled DNA molecules are still present. The fraction of direct double‐strand breaks is 30 ± 5% of all measurable strand cleavage events, is independent of dosage (up to 6.4 GJ/ cm2) and is proportional to In, where I is the average power/area of the 810 nm fs laser pulses, and n = 3 ± 1. The nicking of two DNA strands in the immediate vicinity of the excited PF molecules gives rise to this double‐strand cleavage. In contrast, excitation of the same samples under low‐power, single‐photon absorption conditions (∼400–500 nm) gives rise predominantly to single‐strand breaks, but some double‐strand breaks are observed at the higher dosages. Thus, single‐photon excitation with 400–500 nm light and multiphoton activation of PF by near‐IR fs laser pulses produces different distributions of single‐and double‐strand breaks. These results suggest that DNA strand cleavage originates from unrelaxed, higher excited states when PF is excited by simultaneous IR multiphoton absorption processes.

Multiphoton near-infrared femtosecond laser pulse-induced DNA damage with and without the photosensitizer proflavine.

The excitation of pBr322 supercoiled plasmid DNA with intense near-IR 810 nm fs laser pulses by a simultaneous multiphoton absorption mechanism results in single-strand breaks after treatment of the irradiated samples with Micrococcus luteus UV endonuclease. This enzyme cleaves DNA strands at sites of cyclobutane dimers that are formed by the simultaneous absorption of three (or more) 810 nm IR photons (pulse width approximately 140 fs, 76 MHz pulse repetition, average power output focused through 10x microscope objective is approximately 1.2 MW/cm2). Direct single-strand breaks (without treatment with M. luteus) were not observed under these conditions. However, in the presence of 6 microM of the intercalator proflavine (PF), both direct single- and double-strand breaks are observed under conditions where substantial fractions of undamaged supercoiled DNA molecules are still present. The fraction of direct double-strand breaks is 30 +/- 5% of all measurable strand cleavage events, is independent of dosage (up to 6.4 GJ/cm2) and is proportional to In, where I is the average power/area of the 810 nm fs laser pulses, and n = 3 +/- 1. The nicking of two DNA strands in the immediate vicinity of the excited PF molecules gives rise to this double-strand cleavage. In contrast, excitation of the same samples under low-power, single-photon absorption conditions (approximately 400-500 nm) gives rise predominantly to single-strand breaks, but some double-strand breaks are observed at the higher dosages. Thus, single-photon excitation with 400-500 nm light and multiphoton activation of PF by near-IR fs laser pulses produces different distributions of single- and double-strand breaks. These results suggest that DNA strand cleavage originates from unrelaxed, higher excited states when PF is excited by simultaneous IR multiphoton absorption processes.

IR multiphoton absorption of SF 6 in flow with Ar at moderate energy fluences

in flow with Ar (SF6: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (TV≃230 K, TR≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm2, which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO2-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF6 MPA were studied for several CO2-laser lines which are in a good resonance with the linear absorption spectrum of the ν3 vibration of SF6 at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF6 with Ar was also studied. The results are compared with those obtained in earlier studies.

Collisional effects in the IR multiphoton absorption and dissociation of Si 2F 6

IR multiphoton absorption (MPA) and dissociation (MPD) of Si 2F 6 in Ar were investigated. For the 10R (26) line (979.7 cm −1) of the CO 2 laser, the average number of photons absorbed and the MPD yield were obtained for different laser fluences and buffer gas pressures. The fraction of excited molecules was as high as 70%. The MPD probability was found to be heavily suppressed when more than 5 Torr of Ar buffer gas was added, indicating that vibrational deactivation is predominant over MPA increase with pressure.

IR multiphoton absorption in Si 2F 6

IR multiphoton absorption (IRMPA) in neat Si 2F 6 was measured. The average number of photons absorbed per molecule and the absorption cross-sections are reported as a function of fluence for the 10R(12), 10R(22) and 10R(34) CO 2 laser lines. IRMPA spectra, given in the range from 10R(10) to 10R(38) for three different laser fluences, are discussed and compared with the multiphoton dissociation data.

IR multiphoton absorption and isotopically selective dissociation of CHCIF2 in a herriott multipass cell

The small absorption is a major problem in isotopically selective IR multiphoton dissociation. Usually the radiation diverges before it is fully absorbed. To solve this problem, we have demonstrated the application of a refocusing (Herriott) multipass cell. It can generally help in laser isotope separation to use the photons more efficiently. Employing such a cell and a Q-switched CO2 laser at high repetition rate, the dissociation yield of CHCIF2 was 23 times higher than in a single pass. The number of passes used (up to 60) was more than is conventionally possible in such small cells. The increased number was permitted by making use of spherical aberration. With 18 passes, we also measured the multiphoton absorption for various wavelengths and pressures, in part separately for12CHCIF2 and13CHCIF2, and also for two-wavelength irradiation. Appropriate change of pressure or wavelength increased the absorption. But the corresponding increase of the dissociation was larger in every case. To explain this and other observations, we invoke the molecular distribution over the energy levels.

Dependence of the collisional relaxation of highly vibrationally excited polyatomic molecules on the population distribution function

The influence of the population distribution function on the collisional relaxation of highly vibrationally excited polyatomic molecules is analyzed in a non-reactive system. Unimodal and bimodal energy distributions are considered. Calculations made with unimodal energy distributions showed that the energy decay is almost independent of its initial shape and also of the collisional transition probability models, provided they have the same dependence on energy. When the initial distribution is bimodal, the rate of energy decay, for constant average energy transferred, depends on the fraction of molecules excited, but if the decay is exponential, the energy loss profile is almost independent of the fraction q of molecules excited. These results are discussed in relation to the use of IR multiphoton absorption as an experimental technique for the study of energy transfer processes.

Wavepacket dynamics of hydrogen cyanide excited by intense ultrashort pulsed laser

Dynamics of hydrogen cyanide in the ground electronic state under an intense, impulsive excitation condition is theoretically studied based on the semiclassical theory of the molecule-radiation field interaction. Hydrogen migration in the HCN/HNC system, which originates from a simultaneous, IR-multiphoton absorption induced by such an impulsive excitation, is expressed in terms of the nuclear wavepacket propagation. Dynamic coupling between the CH stretching and HCN bending vibrational modes is shown to play an important role in the hydrogen migration process.

Wavepacket dynamics of hydrogen cyanide excited by intense ultrashort pulsed laser

Dynamics of hydrogen cyanide in the ground electronic state under an intense, impulsive excitation condition is theoretically studied based on the semiclassical theory of the molecule-radiation field interaction. Hydrogen migration in the HCN/HNC system, which originates from a simultaneous, IR-multiphoton absorption induced by such an impulsive excitation, is expressed in terms of the nuclear wavepacket propagation. Dynamic coupling between the CH stretching and HCN bending vibrational modes is shown to play an important role in the hydrogen migration process.

Fluorescence from SF6 in highly excited vibrational states: An overview

The spectroscopy of SF 6 , excited in its quasi-continuum by IR multiphoton absorption, has been recorded by IR fluorescence from ν = 700 to 1300 cm −1 with a highly sensitive device. The far resonance fluorescence exhibits structures directly connected to the intramolecular couplings. A model of couplings has been developed, the parameters of which are fixed by the experimental results. The strongest couplings (orders k = 3 and 4) have a density so low that they produce only a perturbative redistribution of the ν 3 mode. The dissipative redistribution is produced by the highest orders k ⩾5 and can be represented by a half-linewidth γ d ∼1.5 cm −1 for E vib ∼ 15,000 cm −1 . The onset of the SF 6 quasi-continuum is also predicted by the model, with characteristics in full agreement with published data.

Pulsed photoacoustic study of collisional effects in IR multiphoton absorption measurements

The pulsed laser photoacoustic technique, simultaneously used with transmission spectroscopy, was applied for detailed study of IR multiphoton absorption processes in polyatomic molecules. Collisional effects were specially investigated in SF 6 diluted with Ar as nonabsorbing species. The results regarding the enhanced absorption cross sections (σ e ) for SF 6 -Ar mixtures are obtained with precision of ±18-±29%. The differential absorption cross sections (σ d e ), as well as the average number of absorbed photons per SF 6 molecule in 0, 1 -0,7 J/cm 2 fluence range and 1-140 mbar Ar pressure range are also presented

Analysis of CF3I quasicontinuum states. III

CF3I quasicontinuum (QC) states are analyzed in the framework used previously for SF6 in papers I and II of this series. The existence of a hierarchy of intramolecular couplings Vk, decreasing vs the number k of vibrational quanta exchanged, with an effective density of coupled states ρk, is pointed out. The oscillator strength redistribution is described taking into account isolated resonances and perturbative redistribution associated to the lowest orders k, dissipative redistribution associated to higher orders, multiple transitions, and mode inhomogeneous spreading. Published homogeneous spectra of 12CF3I and 13CF3I at E=19 000 cm−1, near the dissociation threshold, are reinvestigated in this framework and a new assignment of the resonances is proposed. Using fluorescence data at 〈E〉=3200 cm−1, an interpolation of the position and weight of the main resonances ν1, ν4, ν2+ν3, 2ν5, in the frequency range 900–1200 cm−1, becomes possible. This interpolation allows to extract from IR multiphoton absorption data, at the laser frequency νL=1072 cm−1, an average Lorentzian half-width γ̄ (〈n〉) vs the average number of photons absorbed 〈n〉. γ̄ increases from 1 to 5 cm−1 for 〈n〉 varying from 5 to 10, and varies slowly around 4–5 cm−1 for 10≤〈n〉≤18. Finally, the onset of the QC, near E∼6000 cm−1, is explained using a modelization of the hierarchy Vk, ρk, without arbitrary parameters, and an intramolecular relaxation time ∼20 ps is predicted. This analysis leads to a very good agreement between the different sets of data concerning the CF3I molecule.

Analysis of SF6 quasicontinuum states. II

The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelié, J. Chem. Phys. 96, 8072 (1992)], are analyzed in comparison with IR multiphoton absorption (IRMPA) data, either in the nanosecond or in the picosecond regimes. A general description of the QC transition matrix elements is first derived. All the following features must be taken into account: multiple rovibrational transitions, perturbative redistribution onto secondary resonances, dissipative redistribution onto the whole energy shell, and intramolecular mode inhomogeneity producing a frequency dispersion. The intramolecular couplings obey a hierarchy Vk versus the number k of quanta exchanged, with an effective density of couplings ρk. A model without arbitrary parameters settles that the parameter γ=πρ&amp;lt;V2≳ can be shared between a dissipative part γd∼1–1.5 cm−1 (for E∼10 000–15 000 cm−1), corresponding to the orders k≥5, and a perturbative part γp∼2–15 cm−1, corresponding to the lowest orders k≂3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E∼4000 cm−1, as demonstrated by Raman experiments, with a half-width γd∼0.21 cm−1, in excellent agreement with picosecond spectroscopy giving a T1 relaxation time of ∼11 ps. The parameter γ is also extracted from IRMPA cross sections. It is found from these data that γ(E) increases from 0.25 to 15 cm−1 when E increases from 4000 to 34000 cm−1. Finally, picosecond data, showing a spectacular enhancement of the number of photons absorbed for short pulses of duration τL∼30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the ν3 mode, producing a dramatic enhancement of the Rabi width. Then, all known data on the SF6 QC can be incorporated in a unique framework, likely generalizable to other molecules.

Intermolecular state couplings in the presence of a radiation field. I. Non-resonant V-V transitions

Non-resonant intermolecular V-V transitions in the presence of an electromagnetic field are explored by using the semiclassical theory and the dressed molecular method. It is found that intense radiation fields can substantially alter the qualitative dynamic behavior of V-V transition processes and hence the transition probabilities. Under favorable conditions, IR multiphoton absorption processes can proceed by overcoming the molecular anharmonicity via collisional V-V transitions between dressed vibration modes. In particular, we have found, from numerical calculations of light-assisted V-V transitions between a CH local mode and CO/CO 2, that if the resonant condition (Δω′=0) is fulfilled, a remarkable enhancement of V-V transition rates and photon absorption rates results in a high-intensity radiation field ( I = 10 12 W/cm 2) which is accessible with present-day laser technology. Furthermore, intermolecular long-range forces may come into play, even become dominant forces in inducing non-resonant V-V transitions, and as a result, V-V transition rates may display a negative temperature dependence in certain temperature ranges. Our study also shows that light-assisted V-V transitions display a quantum interference between dressed collision partners.

Infrared multiphoton excitation of CF3I. II. Collisional energy transfer of vibrationally highly excited CF3I

Energy transfer of vibrationally highly excited CF3I molecules (E≈18 000 cm−1) in collisions with argon, propane, and octane was studied using hot UV absorption spectroscopy of CF3I. the preparation of the excited CF3I was achieved by IR multiphoton absorption which, under the conditions applied, produces a narrow initial energy distribution of CF3I near to the dissociation energy. The average energies 〈ΔE〉 transferred per collision were found to be proportional to E for the bath gases propane and octane; they showed a stronger increase with energy at low excitation energies in the bath gas argon. The energy dependence of 〈ΔE〉, therefore, is not only governed by the properties of the excited molecule (e.g., its density of states) but also by features of the collision partner (e.g., the magnitude of 〈ΔE〉). At energies near to the dissociation energy for all colliders, 〈ΔE〉 was found to approach values similar to those obtained from single UV photon excitation experiments with highly excited triatomic and large polyatomic molecules.

Collisional effects in IR multiphoton absorption of CDF3

Energy absorption by CDF3 for different energy fluences from an intense TEA CO2 laser is reported as function of the pressure of CDF3 and various bath gases (He, Ne, Ar, Xe, and CHF3). Collisional behavior at different conditions is discussed.