Vibrational Population Inversion Research Articles

Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses.

The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800 nm pump laser and a delayed seed laser. It was found that emission lines around both 428 and 391 nm are greatly enhanced upon use of a 391 or 428 nm seed laser. The emission lines around 391 and 428 nm can be assigned to the rotational transitions of N2+ [B2Σu+(v' = 0) → X2Σg+(v = 0)] and N2+ [B2Σu+(v' = 0) → X2Σg+(v = 1)], respectively. They originate from seed-induced superfluorescence and resonant stimulated Raman scattering. The genetic algorithm was utilized to simulate the experimental observations and determine the relative population of B2Σu+(v' = 0), X2Σg+(v = 1), and X2Σg+(v = 0). The result verifies that vibrational population inversion is achieved between B2Σu+(v' = 0) and X2Σg+(v = 0) by the 800 nm pump laser. Our finding provides new insights into controlling the coherent emission of ionized nitrogen molecules, which has promising application in filamentation-based remote atmospheric sensing.

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.

Junction Plasmon Driven Population Inversion of Molecular Vibrations: A Picosecond Surface-Enhanced Raman Spectroscopy Study.

Molecular surface-enhanced Raman spectra recorded at single plasmonic nanojunctions using a 7 ps pulse train exhibit vibrational up-pumping and population inversion. The process is assigned to plasmon-driven, dark, impulsive electron-vibration (e-v) excitation. Both optical (Raman) pumping and hot-electron mediated excitation can be rejected by the characteristic spectra, which allow the simultaneous measurement of vibrational temperature of the molecules and electronic temperature of the metal. Vibrational populations are determined from anti-Stokes to Stokes intensity ratios, while the electron temperature is obtained from the anti-Stokes branch of the electronic Raman scattering continuum. Population inversion survives in high-frequency vibrations that effectively decouple from the metal.

Kinetics and dynamics study of the H + CCl4 → HCl(v′, j′) + CCl3 reaction

Based on a previous potential energy surface describing the H + CCl4 reaction, a new analytical surface named PES-2010 was developed modifying both the functional form to give it more flexibility, and the calibration process in which exclusively theoretical information was used. Thus, the surface is completely symmetric with respect to the permutation of the four methane chlorine atoms, and no experimental information is used in the process. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 300–2,500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is small, since the abstraction reaction involves the motion of a heavy particle (a chlorine atom) that cannot easily tunnel through the reaction barrier. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the HCl stretching mode in the products appears vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid the zero-point energy problem. First, we found that the HCl(ν′, j′) product mostly appears with small rotational energy and vibrational population inversion. Second, the state-specific scattering distributions show backward scattering, which becomes more noticeable as the HCl(ν′) vibrational state increases. Unfortunately, no experimental dynamics data are available for the title reaction, but the comparison with the kinematically similar and well-studied H + Cl2 reaction shows good agreement, indicative of similar mechanisms. These kinetics and dynamics results seem to indicate that the potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment lends further confidence to this new surface.

Three-dimensional quantum mechanical studies of the H + H2 and F + H2 reactions

New results are presented for quantum mechanical reactive scattering in H + H2, including a comparison of close-coupling cross sections for para to ortho conversion with results obtained through momentum decoupling approximations. Information-theoretic analysis of product rotational distributions is given for one total energy. Results are also presented of the first three-dimensional coupled-channel study of the F + H2 reaction, showing the energy dependence of the probability distribution for vibrational excitation and population inversion.

Internal Energy Distributions of OH Products in the Reaction of O(3PJ) with HSiCl3

The OH(X 2 II, υ=0, 1) internal state distributions from the reaction of electronically ground state oxygen atoms with HSiCl 3 were measured using laser-induced fluorescence. The ground-state 0( 3 Pi) atoms with kinetic energies above the reaction barrier were produced by photolysis of NO 2 at 355 nm. The OH product revealed strong vibrational population inversion, P(υ=1)/Pυ=0) = 4.0 ± 0.6, and rotational distributions in both vibrational states exhibit substantial rotational excitations to the limit of total available energy. However, no preferential populations in either of the two A doublet states were observed from the micropopulations, which supports a mechanism involving a direct abstraction of hydrogen by the atomic oxygen. It was also found that the collision energy between O and HSiCl 3 is effectively coupled into the excitation of the internal degrees of freedom of the OH product ( = 0.62, and = 0.20). The dynamics appear consistent with expectations for the kinematically constrained reaction which supports the reaction type, heavy + light-heavy → heavy-light + heavy (H + LH'→ HL + H'). The dynamics of oxygen atom collision with HSiCl 3 are discussed in comparison to those with SiH 4 .

Reactions of Gas-Phase Atomic Hydrogen with Chemisorbed Hydrogen on a Graphite Surface

The reaction of gas-phase hydrogen atoms H with H atoms chemisorbed on a graphite surface has been studied by the classical dynamics. The graphite surface is composed of the surface and 10 inner layers at various gas and surface temperatures (Tg, T s ). Three chains in the surface layer and 13 chains through the inner layers are considered to surround the adatom site. Four reaction pathways are found: H 2 formation, H-H exchange, H desorption, and H adsorption. At (1500 K, 300 K), the probabilities of H 2 formation and H desorption are 0.28 and 0.24, respectively, whereas those of the other two pathways are in the order of 10 -2 . Half the reaction energy deposits in the vibrational motion of H 2 , thus leading to a highly excited state. The majority of the H 2 formation results from the chemisorption-type H(g)-surface interaction. Vibrational excitation is found to be strong for H 2 formed on a cold surface (∼10 K), exhibiting a pronounced vibrational population inversion. Over the temperature range (10-100 K, 10 K), the probabilities of H 2 formation and H-H exchange vary from 0 to ∼0.1, but the other two probabilities are in the order of 10 -3 .

Coherent vibrational climbing in carboxyhemoglobin.

We demonstrate vibrational climbing in the CO stretch of carboxyhemoglobin pumped by midinfrared chirped ultrashort pulses. By use of spectrally resolved pump-probe measurements, we directly observed the induced absorption lines caused by excited vibrational populations up to v = 6. In some cases, we also observed stimulated emission, providing direct evidence of vibrational population inversion. This study provides important spectroscopic parameters on the CO stretch in the strong-field regime, such as transition frequencies and dephasing times up to the v = 6 to v = 7 vibrational transition. We measured equally spaced vibrational transitions, in agreement with the energy levels of a Morse potential up to v = 6. It is interesting that the integral of the differential absorption spectra was observed to deviate far from zero, in contrast to what one would expect from a simple one-dimensional Morse model assuming a linear dependence of dipole moment with bond length.

Dynamics of Hydrogen Molecules Priduced on a Graphite Surface

We have studied the dynamics of energy-rich hydrogen molecules produced on a graphite surface through H(g) + H(ad)/C(gr) → <TEX>$H_2$</TEX> + C(gr) at thermal conditions mimicking the interstellar medium using a classical trajectory procedure. The recombination reaction of gaseous H atom at 100 K and the adsorbed H atom on the interstellar graphite grains at 10 K efficiently takes place on a subpicosecond time scale with most of the reaction exothermicity depositing in the product vibration, which leads to a strong vibrational population inversion. The molecules produced in nearly end-on geometry where H(g) is positioned below H(ad) rotate clockwise and are more highly rotationally excited. but in low-lying vibrational levels. The rotational axis of most of the molecule rotating clockwise is tilted from the surface normal by more than 30°, the intensity peaking at 35°. The molecules produced when H(ad) is close to the surface rotate counter-clockwise and are weakly rotationally excited, but highly vibrationally excited. These molecules tend to align their rotational axes parallel to the surface. The number of molecules rotating clockwise is eight times larger than that rotating counter-clockwise.

Vibrational heating in electron stimulated desorption of CO from transition metals: a classical mechanics analysis

The vibrational population inversion in CO molecules desorbed following electron bombardment of transition metal surfaces is modelled with classical mechanics for a bimodal, two-state model. An excited state potential energy surface obtained from DFT cluster calculations for a doubly excited molecule is modified to examine the effects on the vibrational state resolved desorption probabilities. An interplay of factors is found to determine these probabilities: the potential energy gained from electronic excitation of the molecule, the kinetic energy gained from propagation on the excited state, and the potential energy at the point of relaxation to the electronic ground-state. Within the limited dimensionality of the model, the DFT PES gives a good reproduction of the experimental findings concerning the vibrational population inversion, the translational energy of the desorbing molecules and the positive correlation between vibrational state and translational energy.

Theory of electron stimulated desorption and dissociation of CO at transition metals

Experiments, for the electron stimulated desorption of CO molecules from Ru(0001) [Wurm et al., Phys. Rev. Lett. 74, 2591 (1995)] are rationalized with the help of quantum wave packet methods using a bimodal two-state model. Besides a vibrational population inversion for the desorbing molecule we also find a small amount of dissociation, and an experimentally observed positive correlation between vibrational state and kinetic energy of the desorbing molecules. The role of vibrational excitation of the initial state is discussed. Classical trajectory calculations are found to be in good agreement with quantum dynamics thus allowing for a systematic exploration of the sensitivity of the results on details of the potential energy surfaces.

Dynamics of H 2 formation on a graphite surface

We have studied the dynamics of energy-rich H 2 produced on a graphite surface in H(ad) + H(g) using a classical trajectory procedure. The majority of H(g) recombine with H(ad) on a subpicosecond timescale with a significant portion of the reaction exothermicity depositing in the H 2 vibration, leading to a vibrational population inversion. Vibrational excitation is strong when H(g) first approaches close proximity of the surface and rebounds toward H(ad) in a direct-mode mechanism. H(g) reaching close to the surface at large impact parameter tends to become trapped before rebounding to abstract H(ad), a complex-mode mechanism.

Inverted vibrational distributions from N2 recombination at Ru(001): Evidence for a metastable molecular chemisorption well

We have measured translational and internal state distributions for N2 desorbed from a Ru(001) surface following NH3 cracking at 900 K. Nitrogen is formed with a vibrational population inversion, P(v=1)/P(v=0)=1.4, but a subthermal rotational energy release, Trot(v=0)=630 K. The translational energy distributions show a peak at low energy with a tail extending up to ∼2 eV and a mean energy release of 0.62 eV for N2(v=0) and 0.61 eV for (v=1). The product state distributions indicate a preferential energy release into the N2 stretching coordinate with a relatively weak N2–surface repulsion. Density functional calculations for N2 dissociation on Ru(001) and Cu(111) have been performed to compare the shape of the potentials in the N2 stretching (d) and translational (Z) coordinates. These reveal a sharp curvature of the surface for Ru, the energy release occurring close to the surface over a narrow range of Z. We suggest that this behavior is the result of the presence of a metastable molecular state, bound close to the surface with a short N2 bond, as predicted by Mortensen et al. [J. Catalysis, 169, 85 (1997)]. We contrast the dynamics on Ru with that observed for N recombination on Cu(111) [Murphy et al., J. Chem. Phys. 109, 3619 (1998)], where the potential energy surface shows no evidence for a molecular chemisorption well. Detailed balance arguments predict that N2 dissociation on Ru(001) is highly activated, S(E) increasing by nine orders of magnitude between 0.1 and 2 eV translational energy. The vibrational population inversion implies that vibration promotes dissociation more efficiently than translational excitation, sticking having a vibrational efficacy of 1.3. The predicted S(E) are consistent with reports of a very low sticking probability (S&amp;lt;10−9) on Ru(001) at thermal energies but in disagreement with recent molecular beam adsorption measurements.

Reaction of gas phase atomic oxygen with chemisorbed hydrogen atoms on a tungsten surface

Classical trajectory studies have been carried out for the reaction of gas phase atomic O with H atoms chemisorbed on a corrugated tungsten surface. Most reactive events are found to occur in single-impact collisions on a subpicosecond scale via the Eley-Rideal mechanism producing highly excited OH. The extent of reaction is strongly affected by surface corrugation and product is shown to exhibit a vibrational population inversion. The direct coupling of the reaction zone to the solid atom chain consisting of nine atoms is considered to obtain consistent results. A LEPS potential energy surface is used for the reaction zone interaction.

Reaction of atomic oxygen with adsorbed carbon monoxide on a platinum surface

The reaction of gas-phase oxygen atoms with carbon monoxide molecules adsorbed on a platinum surface is studied by the use of the classical trajectory approach. Collisions taking place at gas temperature 300 K are considered as a function of the incident angle. Gas atoms approaching CO in the angle range of 0°–50° are very efficient at producing vibrationally excited CO2 molecules in the gas phase. Beyond 50°, the extent of desorbing CO2 formation decreases rapidly and becomes negligible as the incident angle approaches 90°. Most of the exothermicity of the reaction O+CO→CO2 minus the CO–surface-binding energy appears to be transferred to the asymmetric stretching vibration of the desorbing CO2. The fraction of reactive collisions producing molecules having vibrational energies corresponding to levels v3=9 to 13 is found to be very high and exhibits a vibrational population inversion. Molecular time scale trajectory calculations show that relatively few atoms making up the solid are needed to obtain reliable data on energy transfer to the solid. The behavior of ensembles at various reaction times is discussed in detail. The surface is considered to be at 0 K.

Vibrational population inversion in CS(A) through the dissociative excitation of CS2 by Ar(3P2)

The CS(A 1Π → X 1Σ+) emission spectra resulting from the energy transfer reaction of Ar(3P2) + CS2 under single collision condition have been obtained. The relative vibrational populations of the nascent product CS(A 1Π, υ′) have been determined by means of spectral simulation. A population inversion is found at υ′ = 1. The population data are approximately represented by a distribution predicted from the impulsive half collision model. The dynamics and energetics of CS(A) formation has been discussed in detail.

Rate constants for CN reactions with hydrocarbons and the product HCN vibrational populations: Examples of heavy–light–heavy abstraction reactions

The rate constants for the reactions of CN radicals with methane, ethane, propane, cyclo-propane, isobutane, and neopentane have been measured over a temperature range from 275 to 455 K. Laser photolysis was used to produce the radicals and time delayed laser induced fluorescence was used to follow the radical concentration as a function of time. The temperature dependence of the observed rate constants could be fitted with a three-parameter Arrhenius plot. The activation energies that were observed were all small and in some cases they were negative. Time resolved ir emission was used to follow the formation of the HCN(0n2) and HCN(0n′1) product emission. The time dependence of the relative emission intensities, as well as computer modeling of the decay curves, suggest that vibrational population inversion occurs for all of the hydrocarbons studied except methane and cyclopropane. These observations are discussed in terms of the current theories for these type of reactions.

Vibrational population inversion in aniline following trapping/desorption from fullerene surfaces

Aniline molecules were state selectively detected using multiphoton ionization following trapping/desorption from amorphous C 60 and C 70 films, and from single-crystal C 60 surfaces. Vibrational population inversion was observed in all three cases in the inversion mode of the NH 2 group. The vibrational excitation was much less efficient for scattering from a LiF single-crystal surface. A charge-transfer model rationalizes the observations and indicates that the fullerene surfaces strongly interacts with molecules with low ionization potentials

Product internal-state distribution for the reaction H+HI→H2+I

We have measured the nascent H2(v, j) product-state distribution from the H+HI→H2+I abstraction reaction. Laser photolysis of HI at 266 nm generated translationally hot H atoms with center-of-mass collision energies of 1.61 and 0.68 eV in the ratio 64:36. Quantum-state-specific detection of the molecular reaction product was accomplished via (2+1) resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry. The H2 is formed with a high degree of internal excitation, including a vibrational population inversion between v=0 and v=1. Our product-state distribution agrees closely with that of Aker, Germann, and Valentini where comparison is possible. Rotational population distributions derived from the quasiclassical trajectory calculations of González and Sayós are generally too cold, whereas those of Aker and Valentini nearly reproduce the experimental distributions. Both calculations fail to predict, however, the observed vibrational inversion.

The production of OH( 2Π) in the reaction O( 3P)+GeH 4

The OH( v=0 and 1) product rotational- and fine-structure-state distributions from the reaction O( 3P)+GeH 4 have been determined by laser-induced fluorescence in a pulse-probe experiment. There is strong vibrational population inversion P( v=1)/ P( v=0)=4.2±2.4. The rotational-state distributions extend to high rotational levels and are non-thermal with non-statistical lambda-doublet state populations suggesting that OH is formed in these vibrational levels by an addition mechanism rather than by direct H atom abstraction.