Rotationally resolved IR-diode laser studies of ground-state CO2 excited by collisions with vibrationally excited pyridine

Relaxation of highly vibrationally excited pyridine (C5NH5) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyridine (E' = 40,660 cm(-1)) was prepared by 248 nm excimer laser excitation followed by rapid radiationless relaxation to the ground electronic state. Pyridine then collides with CO2, populating the high rotational CO2 states with large amounts of translational energy. The CO2 nascent rotational population distribution of the high-J (J = 58-80) tail of the 00(0)0 state was probed at short times following the excimer laser pulse to measure rate constants and probabilities for collisions populating these CO2 rotational states. Doppler spectroscopy was used to measure the CO2 recoil velocity distribution for J = 58-80 of the 00(0)0 state. The energy-transfer distribution function, P(E,E'), from E' - E approximately 1300-7000 cm(-1) was obtained by re-sorting the state-indexed energy-transfer probabilities as a function of DeltaE. P(E,E') is fit to an exponential or biexponential function to determine the average energy transferred in a single collision between pyridine and CO2. Also obtained are fit parameters that can be compared to previously studied systems (pyrazine, C6F6, methylpyrazine, and pyrimidine/CO2). Although the rotational and translational temperatures that describe pyridine/CO2 energy transfer are similar to previous systems, the energy-transfer probabilities are much smaller. P(E,E') fit parameters for pyridine/CO2 and the four previously studied systems are compared to various donor molecular properties. Finally, P(E,E') is analyzed in the context of two models, one indicating that P(E,E') shape is primarily determined by the low-frequency out-of-plane donor vibrational modes, and the other that indicates that P(E,E') shape can be determined from how the donor molecule final density of states changes with DeltaE.     

Quenching of highly vibrationally excited pyrimidine by collisions with CO2

Relaxation of highly vibrationally excited pyrimidine (C(4)N(2)H(4)) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyrimidine (E(')=40 635 cm(-1)) was prepared by 248-nm excimer laser excitation, followed by rapid radiationless relaxation to the ground electronic state. The nascent rotational population distribution (J=58-80) of the 00(0)0 ground state of CO(2) resulting from collisions with hot pyrimidine was probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the CO(2) recoil velocity distribution for J=58-80 of the 00(0)0 state. Rate constants and probabilities for collisions populating these CO(2) rotational states were determined. The measured energy transfer probabilities, indexed by final bath state, were resorted as a function of DeltaE to create the energy transfer distribution function, P(E,E(')), from E(')-E approximately 1300-7000 cm(-1). P(E,E(')) is fitted to a single exponential and a biexponential function to determine the average energy transferred in a single collision between pyrimidine and CO(2) and parameters that can be compared to previously studied systems using this technique, pyrazineCO(2), C(6)F(6)CO(2), and methylpyrazineCO(2). P(E,E(')) parameters for these four systems are also compared to various molecular properties of the donor molecules. Finally, P(E,E(')) is analyzed in the context of two models, one which suggests that the shape of P(E,E(')) is primarily determined by the low-frequency out-of-plane donor vibrational modes and one which suggests that the shape of P(E,E(')) can be determined by how the donor molecule final density of states changes with DeltaE.     

Kinetically controlled selective ionization study on the efficient collisional energy transfer in the deactivation of highly vibrationally excited trans-stilbene

Direct measurements of the gas-phase collisional energy transfer parameters are reported for the deactivation of highly vibrationally excited trans-stilbene molecules, initially prepared with an average energy of about 40 000 cm(-1), in the bath gases argon, CO2, and n-heptane. The method of kinetically controlled selective ionization (KCSI) has been used. Complete experimental collisional transition probability density functions P(E',E) are determined, which are represented by a monoexponential form with a parametric exponent in the argument, P(E',E) proportional to exp[-{(E - E')/(C0 + C1E)}Y] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, kE,1, or of resulting first moments of energy transfer, , for trans-stilbene with those for azulene and toluene clearly shows the considerably more efficient deactivation of trans-stilbene for all bath gases, presumably due to the much greater number of very low-frequency modes of trans-stilbene. However, on a relative scale this gain in deactivation rate of excited trans-stilbene is clearly collider dependent and decreases distinctly with the growing collision efficiency of the larger bath gas molecules.     

Energy transfer between polyatomic molecules II: Energy transfer quantities and probability density functions in benzene, toluene, p-xylene, and azulene collisions

Collisional energy transfer, CET, is of major importance in chemical, photochemical, and photophysical processes in the gas phase. In Paper I of this series (J. Phys. Chem. B 2005, 109, 8310) we have reported on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. In the present work, we report on CET between excited toluene, p-xylene, and azulene with cold benzene and Ar and on CET of excited benzene with cold toluene, p-xylene, and azulene. We compare our results with those of Paper I and report average vibrational, rotational, and translational energy quantities, , transferred in a single collision. We discuss the effect of internal rotation on CET and the identity of the gateway modes in CET and the relative role of vibrational, rotational, and translational energies in the CET process, all that as a function of temperature and excitation energy. Energy transfer probability density functions, P(E,E'), for the various systems are reported and the shape of the curves for various systems and initial conditions is discussed. The major findings for polyatomic-polyatomic collisions are: CET takes place mainly via vibration-to-vibration energy transfer assisted by overall rotations. Internal free rotors in the excited molecule hinder energy exchange while in the bath molecule they do not. Energy transfer at low temperatures and high temperatures is more efficient than that at intermediate temperatures. Low-frequency modes are the gateway modes for energy transfer. Vibrational temperatures affect energy transfer. The CET probability density function, P(E,E'), is convex at low temperatures and can be concave at high temperatures. A mechanism that explains the high values of and the convex shape of P(E,E') is that in addition to short impulsive collisions there are chattering collisions where energy is transferred in a sequence of short encounters during the lifetime of the collision complex. This also leads to the observed supercollision tail at the down wing of P(E,E'). Polyatomic-Ar collisions show mechanistic similarities to polyatomic-polyatomic collisions, but there are also many dissimilarities: internal rotations do not inhibit energy transfer, P(E,E') is concave at all temperatures, and there is no contribution of chattering collisions.     

Trajectory study of supercollision relaxation in highly vibrationally excited pyrazine and CO2

Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.     

Relaxation dynamics of highly vibrationally excited picoline isomers (E(vib) = 38 300 cm(-1)) with CO2: the role of state density in impulsive collisions

Strong collisions of highly vibrationally excited picoline isomers and CO2 (00(0)0) were investigated using high resolution transient IR absorption probing to investigate the role of donor state density. Vibrationally excited 3-picoline and 4-picoline (3-methylpyridine and 4 methylpyridine) with E(vib) = 38300 cm(-1) were prepared by 266 nm excitation followed by rapid internal conversion. Transient IR probe measurements of the nascent rotational and translational energy gain in CO2 (00(0)0) show that large DeltaE collisions for 3- and 4-picoline are similar to those for excited 2-picoline. The probability distributions for the large DeltaE energy transfer of the three isomers have similar dependence on DeltaE. The results are compared with other earlier results demonstrating that the shape of the large DeltaE probability distribution correlates with the DeltaE dependence of the donor vibrational state density. The results are discussed in terms of the GRETCHEN model for collisional relaxation.     

Energy transfer between polyatomic molecules. 1. Gateway modes, energy transfer quantities and energy transfer probability density functions in benzene-benzene and Ar-benzene collisions

We report collisional energy transfer, CET, quantities for polyatomic-polyatomic collisions and use excited benzene collisions with cold benzene bath, B-B, as our sample system and compare our results with the CET of excited benzene with Ar bath. We find that the gateway mode for both systems is the out-of-plane modes and that in B-B CET, vibration to vibration, V-V, is the dominant channel. Rotations play a mechanistic role in the CET but the net rotational energy transfer is small compared to V-V. The shape of the down side of the energy transfer probability density function, P(E,E'), is convex for B-B collisions and it becomes less so as the temperature increases. In Ar-B collisions, P(E,E') is concave and it becomes less so as the temperature decreases. We report average vibrational, rotational, and translational energy transferred, , as function of temperature for various initial conditions.     

Accounting for the dependence of P(E',E) on the maximum impact parameter in classical trajectory calculations: application to the H2O-H2O collisional relaxation

In this work we report a novel methodology that is able to predict how energy transfer transition probability density functions [P(E',E)] change with the maximum impact parameter (bmax) used in trajectory calculations (TC's). The method assumes that P(E',E) can be described by a sum of exponential functions and that all the trajectories with an initial impact parameter beyond a certain critical value will contribute only to the elastic peak [P(E',E) for E'=E]. This approach is applied to H2O-H2O collisions at different initial vibrational energies of the excited molecules and temperatures of bath gas. The results show that it is possible to reproduce with high accuracy the whole P(E',E) obtained from a given bmax, using the results of TC's performed at another bmax. The new methodology also leads us to propose a new criterion to choose the value of bmax.     

Alkylation effects on strong collisions of highly vibrationally excited alkylated pyridines with CO2

The role of alkylation on the energy partitioning in strong collisions with CO2 was investigated for highly vibrationally excited 2-ethylpyridine (2EP) and 2-propylpyridine (2PP) prepared with E(vib) approximately 38,570 and 38,870 cm(-1), respectively, using lambda = 266 nm light. Nascent energy gain in CO2 (00(0)0) rotation and translation was measured with high-resolution transient absorption spectroscopy at lambda approximately 4.3 microm and the results are compared to earlier relaxation studies of pyridine (E(vib) = 37,950 cm(-1)) and 2-methylpyridine (2MP, Evib = 38,330 cm(-1)). Overall, the alkylated donors impart less rotational and translational energy to CO2 than does pyridine. 2PP consistently imparts more translational energy in collisions than does 2EP and has larger energy transfer rates. Of the alkylated donors, 2MP and 2PP have larger probabilities for strong collisional energy transfer than does 2EP. Two competing processes are discussed: donors with longer alkyl chains have lower average energy per mode and fewer strong collisions but longer alkyl chains increase donor flexibility, leading to higher state densities that enhance energy loss via strong collisions. A comparison of state density effects based on Fermi's Golden Rule shows that 2PP has more strong collisions than predicted while 2EP has fewer. The role of torsional motion in the hot donors is considered. Comparison of effective impact parameters shows that the alkylated donors undergo strong collisions with CO2 via a less repulsive part of the intermolecular potential than does pyridine.     

Energy transfer between azulene and krypton: comparison between experiment and computation

Trajectory calculations of collisional energy transfer between excited azulene and Kr are reported, and the results are compared with recent crossed molecular beam experiments by Liu et al. [J. Chem. Phys. 123, 131102 (2005); 124, 054302 (2006)]. Average energy transfer quantities are reported and compared with results obtained before for azulene-Ar collisions. A collisional energy transfer probability density function P(E,E'), calculated at identical initial conditions as experiments, shows a peak at the up-collision branch of P(E,E') at low initial relative translational energy. This peak is absent at higher relative translational energies. There is a supercollision tail at the down-collision side of the probability distribution. Various intermolecular potentials are used and compared. There is broad agreement between experiment and computation, but there are some differences as well.     

Statistical theory of collisional energy transfer in molecular collisions. trans-stilbene deactivation by argon, carbon dioxide, and n-heptane

Recent advances in experimental techniques have made it possible to measure the full conditional probability density P(E, E') of the energy transfer between two colliding molecules in the gas phase, one of which is highly energized and the other in thermal equilibrium at a given temperature. Data have now become available for trans-stilbene deactivation by the three bath gas molecules Ar, CO2, and n-heptane (C7H16). The initial energies of trans-stilbene are set to 10 000, 20 000, 30 000, and 40 000 cm (-1). The results show that exceptionally large amounts of energy are transferred in each collision. By application of our partially ergodic collision theory (PECT), we find that the energy transfer efficiency betaE ranges from a rather normal value of 0.15 for n-heptane at the highest excitation energy to 0.93-nearly in the ergodic collision limit-for the argon bath gas at high excitation energy. Generally, the PECT produces a good fit of the data except for the nearly elastic peak in the case of n-heptane, where PECT produces a rounded and downshifted peak in contrast to a sharply defined elastic maximum of the monoexponential functional fit produced from the original experimental data obtained by kinetically controlled selective ionization in the work of the group of Luther in G?ttingen. This problem is analyzed and found to be related partly to the lack of treatment of glancing collisions in the theory with a remaining uncertainty due to the weak dependence of energy transfer efficiency on nearly elastic collisions. A summary of the present state of understanding shows that collisional activation and deactivation of reactant molecules is more efficient and more statistical than has been previously realized.     

Dynamics of energy transfer in peptide-surface collisions

Classical trajectory simulations are performed to study energy transfer in collisions of protonated triglycine (Gly)(3) and pentaglycine (Gly)(5) ions with n-hexyl thiolate self-assembled monolayer (SAM) and diamond [111] surfaces, for a collision energy E(i) in the range of 10-110 eV and a collision angle of 45 degrees. Energy transfer to the peptide ions' internal degrees of freedom is more efficient for collision with the diamond surface; i.e., 20% transfer to peptide vibration/rotation at E(i) = 30 eV. For collision with diamond, the majority of E(i) remains in peptide translation, while the majority of the energy transfer is to surface vibrations for collision with the softer SAM surface. The energy-transfer efficiencies are very similar for (Gly)(3) and (Gly)(5). Constraining various modes of (Gly)(3) shows that the peptide torsional modes absorb approximately 80% of the energy transfer to the peptide's internal modes. The energy-transfer efficiencies depend on E(i). These simulations are compared with recent experiments of peptide SID and simulations of energy transfer in Cr(CO)(6)(+) collisions with the SAM and diamond surfaces.     

PECT model analysis and predictions of experimental collisional energy transfer probabilities P(E',E) and moments <DeltaE> for azulene and biphenylene

Experimental collisional energy transfer data from kinetically controlled selective ionization (KCSI) and ultraviolet absorption (UVA) experiments are analyzed in the framework of the partially ergodic collision theory (PECT). Collisions of azulene and biphenylene with different colliders are investigated as case studies. The downward wings of the P(E',E) energy transfer distributions obtained from the PECT model are fitted to the recently introduced "variable-shape"-exponential 3-parameter functional form of P(E',E) obtained from KCSI experiments, P(E',E) proportional, variant exp[-{(E - E')/(C(0) + C(1)E)}(Y)]. The PECT model is able to reproduce the characteristic dependence of the KCSI "shape parameter" Y on the choice of collider, the energy dependent width of the KCSI P(E',E) distributions, described by alpha(E) = C(0) + C(1)E, and the temperature dependence of the UVA data above room temperature. The statistical approach of PECT obviously captures the essence of large molecule energy transfer at chemically significant energies without the need of knowing specific features of the detailed collision dynamics. It therefore shows promise for predicting the shape of P(E',E) in master equation kernels for larger molecules.     

Energy transfer between polyatomic molecules. 3. Energy transfer quantities and probability density functions in self-collisions of benzene, toluene, p-xylene and azulene

This paper is the third and last in a series of papers that deal with collisional energy transfer, CET, between aromatic polyatomic molecules. Paper 1 of this series (J. Phys. Chem. B 2005, 109, 8310) reports on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. Paper 2 in the series (J. Phys. Chem., in press) discusses CET between excited toluene, p-xylene and azulene with cold benzene and Ar and CET between excited benzene colliding with cold toluene, p-xylene and azulene. The present work reports on CET in self-collisions of benzene, toluene, p-xylene and azulene. Two modes of excitation are considered, identical excitation energies and identical vibrational temperatures for all four molecules. It compares the present results with those of papers 1 and 2 and reports new findings on average vibrational, rotational, and translational energy, , transferred in a single collision. CET takes place mainly via vibration to vibration energy transfer. The effect of internal rotors on CET is discussed and CET quantities are reported as a function of temperature and excitation energy. It is found that the temperature dependence of CET quantities is unexpected, resembling a parabolic function. The density of vibrational states is reported and its effect on CET is discussed. Energy transfer probability density functions, P(E,E'), for various collision pairs are reported and it is shown that the shape of the curves is convex at low temperatures and can be concave at high temperatures. There is a large supercollision tail at the down wing of P(E,E'). The mechanisms of CET are short, impulsive collisions and long-lived chattering collisions where energy is transferred in a sequence of short internal encounters during the lifetime of the collision complex. The collision complex lifetimes as a function of temperature are reported. It is shown that dynamical effects control CET. A comparison is made with experimental results and it is shown that good agreement is obtained.     

Building transition probabilities for any condition using reduced cumulative energy transfer functions in H2O-H2O collisions

The energy transfer process between highly vibrationally excited H(2)O in thermal equilibrium with a gas bath of H(2)O at different internal energies and temperatures has been studied by classical trajectory calculations. The results were analyzed using a cumulative probability distribution Q(DeltaE) of the amount of energy transferred, obtained by direct count of the number of trajectories that transfer an amount of energy equal to or greater than a certain value DeltaE. Scaling Q(DeltaE) in terms of the mean down and up energies transferred for each group of trajectories results in a unique distribution. This fact and the use of detailed balance constrains were used to propose a methodology that make it possible to build the whole P(E('),E) for any condition by knowing DeltaE and a series of parameters that depend only on the system under study.     

Rotational and vibrational relaxation of methane excited to 2nu3 in CH4/H2 and CH4/He mixtures at 296 and 193 K from double-resonance measurements

A series of time-resolved IR-IR double-resonance experiments have been conducted where methane molecules are excited into a selected rovibrational level of the 2nu3(F2) vibrational substate of the tetradecad and where the time evolution of the population of the various energy levels is probed by a tunable continuous wave laser. The rotational relaxation and vibrational energy transfer processes occurring in methane upon inelastic CH4-H2 and CH4-He collisions have been investigated by this technique at room temperature and at 193 K. By probing transitions in which either the lower or the upper level is the laser-excited level, rotational depopulation rates in the 2nu3(F2) substate were measured. The rate constants for CH4-H2 collisions were found to be 17.7 +/- 2.0 and 18.9 +/- 2.0 micros(-1) Torr(-1) at 296 and 193 K, respectively, and for CH(4)-He collisions they are 12.1 +/- 1.5 and 16.0 +/- 2.0 micros(-1) Torr(-1) at the same temperatures. The vibrational relaxation was investigated by probing other stretching transitions such as 2nu3(F2) - nu3, nu3 + 2nu4 - 2nu4, and nu3 + nu4 - nu4. A kinetic model, taking into account the main collisional processes connecting energy levels up to 6000 cm(-1), that has been developed to describe the various relaxation pathways allowed us to calculate the temporal evolution of populations in these levels and to simulate double-resonance signals. The different rate coefficients of the vibrational relaxation processes involved in these mixtures were determined by fitting simulated signals to the observed signals corresponding to assigned transitions. For vibration to translation energy transfer processes, hydrogen is a much more efficient collision partner than helium, nitrogen, or methane itself at 193 K as well as at room temperature.     

Rovibrational energy transfer in the 4nuCH manifold of acetylene, viewed by IR-UV double resonance spectroscopy. 5. Detailed kinetic model

Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy provides a distinctive way to examine collision-induced state-to-state energy transfer between rotational J-levels in vibrational manifolds of small polyatomic molecules, such as acetylene (C2H2) in its electronic ground state X. We consider the 4nuCH rovibrational manifold of C2H2 at approximately 12,700 cm(-1), where the principal source of IR-brightness is the (nu1+3nu3) or (1 0 3 0 0)0 Sigma+u vibrational combination level. In this highly congested manifold, anharmonic, l-resonance, and Coriolis couplings affect the J-levels of interest, implicating them in a complicated variety of intramolecular dynamics. Previous papers of this series have reported several seemingly anomalous J-resolved phenomena induced by collisions in C2H2 gas at room temperature with pressures and IR-UV pump-probe delay intervals corresponding to remarkably high Lennard-Jones collisional efficiencies P: odd-DeltaJ rotational energy transfer (10(-3)相似文献   

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)相似文献