High-resolution Transient IR Absorption Research Articles

Transient IR spectroscopy of optically centrifuged CO2 (R186-R282) and collision dynamics for the J = 244-282 states.

Collisions of optically centrifuged CO2 molecules with J = 244-282 (Erot = 22 800-30 300 cm-1) are investigated with high-resolution transient IR absorption spectroscopy to reveal collisional and orientational phenomena of molecules with hyper-thermal rotational energies. The optical centrifuge is a non-resonant optical excitation technique that uses ultrafast, 800 nm chirped pulses to drive molecules to extreme rotational states through sequential Raman transitions. The extent of rotational excitation is controlled by tuning the optical bandwidth of the excitation pulses. Frequencies of 30 R-branch ν3 fundamental IR probe transitions are measured for the J = 186-282 states of CO2, expanding beyond previously reported IR transitions up to J = 128. The optically centrifuged molecules have oriented angular momentum and unidirectional rotation. Polarization-sensitive transient IR absorption of individual rotational states of optically centrifuged molecules and their collision products reveals information about collisional energy transfer, relaxation kinetics, and dynamics of rotation-to-translation energy transfer. The transient IR probe also measures the extent of polarization anisotropy. Rotational energy transfer for lower energy molecules is discussed in terms of statistical models and a comparison highlights the role of increasing energy gap with J and angular momentum of the optically centrifuged molecules.

Rotational energy transfer kinetics of optically centrifuged CO molecules investigated through transient IR spectroscopy and master equation simulations.

A combined experimental and theoretical study of quantum state-resolved rotational energy transfer kinetics of optically centrifuged CO molecules is presented. In the experiments, inverted rotational distributions of CO in rotational states up to J = 80 were prepared using two different optical centrifuge traps, one with the full spectral bandwidth of the optical centrifuge pulses, and one with reduced bandwidth. The relaxation kinetics of the high-J tail of the inverted distribution from each optical trap was determined based on high-resolution transient IR absorption measurements. In parallel studies, master equation simulations were performed using state-to-state rate constants for CO-CO collisions in states up to J = 90, based on data from double-resonance experiments for CO with J = 0-29 and a fit to a statistical power exponential gap model. The model is in qualitative agreement with the observed relaxation profiles, but the observed decay rate constants are smaller than the simulated values by as much as a factor of 10. The observed decay rate constants also have a stronger J-dependence than predicted by the model. The results are discussed in terms of angular momentum and energy conservation, and compared to the observed orientational anisotropy decay kinetics of optically centrifuged CO molecules. Models for rotational energy transfer could be improved by including angular momentum effects.

State-resolved rotational distributions and collision dynamics of CO molecules made in a tunable optical centrifuge.

State-resolved distributions and collision dynamics of optically centrifuged CO molecules with orientated angular momentum are investigated by probing the CO J = 29-80 rotational levels using high-resolution transient IR absorption spectroscopy. An optical centrifuge with tunable bandwidth is used to control the extent of rotational excitation in the sample. The rotational distributions are inverted with a maximum population in J = 62. Rotational levels with J > 62 have populations that correlate with the intensity profile of the optical trap. The full bandwidth trap excites CO up to the J = 80 level, while J = 67 is the highest level observed in the reduced bandwidth trap. Polarization-sensitive transient spectroscopy shows that the initial orientational anisotropy is r = 0.8 for levels with J ≥ 55, while anisotropy values are near r = 0.4 for levels with J < 50. The rotational distribution for J > 50 is broadened slightly by collisions, consistent with small |ΔJ| propensity rules for rotational energy transfer. Doppler-broadened line profiles show that the J = 60-80 levels have translational temperatures near Ttrans = 300 K and that these temperatures remain constant for as much as 24 gas kinetic collisions. Doppler linewidths for levels with J < 60 are broadened by non-resonant rotation-to-translation energy transfer. Kinetic analysis of transient signals shows that collisions with thermal bath molecules are the predominant relaxation pathway.

Transient IR (0001–0000) absorption spectroscopy of optically centrifuged N2O with extreme rotation up to J = 205

High-resolution transient IR absorption spectroscopy was used to measure (0001− 0000) R-branch transition frequencies for N2O molecules in extreme rotational states, with quantum number up to J = 205 and energies as high as Erot=17,000 cm−1. A population inversion of rotationally excited N2O states was prepared with an optical centrifuge and probed in a multi-pass IR cell using a quantum cascade laser. The optical centrifuge is based on 800-nm ultrafast, chirped laser pulses that optically trap molecules and accelerate them angularly to extreme rotational states. This work substantially increases the range of observed transitions for this band beyond the J = 100 transitions previously reported and provides benchmark measurements for theoretical predictions based on an effective Hamiltonian using a polyad model. Transient Doppler-broadened IR line profiles of N2O show that optical excitation of the sample is selectively partitioned into rotation, prior to Doppler broadening from rotation-to-translation collisional energy transfer. These results demonstrate how high-J transitions can be measured without thermal heating by coupling optical centrifuge excitation with high-resolution transient IR absorption probing.

Importance of rotational adiabaticity in collisions of CO2 super rotors with Ar and He.

The collision dynamics of optically centrifuged CO2 with Ar and He are reported here. The optical centrifuge produces an ensemble of CO2 molecules in high rotational states (with J ∼ 220) with oriented angular momentum. Polarization-dependent high-resolution transient IR absorption spectroscopy was used to measure the relaxation dynamics in the presence of Ar or He by probing the CO2 J = 76 and 100 states with Erot=2306 and 3979 cm-1, respectively. The data show that He relaxes the CO2 super rotors more quickly than Ar. Doppler-broadened line profiles show that He collisions induce substantially larger rotation-to-translation energy transfer. CO2 super rotors have greater orientational anisotropy with He collisions and the anisotropy from the He collisions persists longer than with Ar. Super rotor relaxation dynamics are discussed in terms of mass effects related to classical gyroscope physics and collisional rotational adiabaticity.

Anisotropic kinetic energy release and gyroscopic behavior of CO2 super rotors from an optical centrifuge.

An optical centrifuge is used to generate an ensemble of CO2 super rotors with oriented angular momentum. The collision dynamics and energy transfer behavior of the super rotor molecules are investigated using high-resolution transient IR absorption spectroscopy. New multipass IR detection provides improved sensitivity to perform polarization-dependent transient studies for rotational states with 76 ≤ J ≤ 100. Polarization-dependent measurements show that the collision-induced kinetic energy release is spatially anisotropic and results from both near-resonant energy transfer between super rotor molecules and non-resonant energy transfer between super rotors and thermal molecules. J-dependent studies show that the extent and duration of the orientational anisotropy increase with rotational angular momentum. The super rotors exhibit behavior akin to molecular gyroscopes, wherein molecules with larger amounts of angular momentum are less likely to change their angular momentum orientation through collisions.

Impulsive Collision Dynamics of CO Super Rotors from an Optical Centrifuge.

We report state-resolved collision dynamics for CO molecules prepared in an optical centrifuge and measured with high-resolution transient IR absorption spectroscopy. Time-resolved polarization-sensitive measurements of excited CO molecules in the J=29 rotational state reveal that the oriented angular momentum of CO rotors is relaxed by impulsive collisions. The translational energy gains for molecules in the initial plane of rotation are threefold larger than for randomized angular momentum orientations, indicating the presence of anisotropic kinetic energy. The transient data show enhanced population for CO molecules in the initial plane of rotation immediately following the optical centrifuge pulse. A comparison with previous CO2 super rotor studies illustrates the behavior of molecular gyroscopes; spatial reorientation of CO2 J=76 rotors takes substantially longer than that for CO J=29 rotors, despite similarities in classical rotational period and rotational energy gap. High-resolution transient IR absorption measurements of the CO J=29-39 rotational states show that the collisional depopulation rates increase with J quantum number.

State-Specific Collision Dynamics of Molecular Super Rotors with Oriented Angular Momentum.

An optical centrifuge pulse drives carbon dioxide molecules into ultrahigh rotational states with rotational frequencies of ω ≈ 32 THz based on the centrifuge frequency at the full width at half-maximum of the spectral chirp. High-resolution transient IR absorption spectroscopy is used to measure the time-evolution of translational and rotational energy for a number of states in the range of J = 0-100 at a sample pressure of 5-10 Torr. Transient Doppler profiles show that the products of super rotor collisions contain substantial amounts of translational energy, with J-dependent energies correlating to a range of ΔJ propensities. The transient population in J = 100 is short-lived, indicating rapid relaxation of high J states; populations in J = 36, 54, and 76 increase overall as the super rotor energy is redistributed. Transient line profiles for J = 0 and 36 are consistently narrower than the initial ambient sample temperature, showing that collision cross sections for super rotors increase with decreasing collision energy. Quantum scattering calculations on Ar-CO2(j) collisions are used to interpret the qualitative features of the experimental results. The results of this study provide the groundwork for developing a more complete understanding of super rotor dynamics.

Full State-Resolved Energy Gain Profiles of CO2 (J = 2−80) from Collisions of Highly Vibrationally Excited Molecules. 1. Relaxation of Pyrazine (E = 37900 cm−1)

State-resolved energy gain profiles for "strong" and "weak" collisions of pyrazine (E = 37900 cm(-1)) and CO(2) are reported. Nascent energy profiles for scattered CO(2) (00(0)0, J = 2-64) were measured using high-resolution transient IR absorption spectroscopy at lambda = 2.7 microm. The data are combined with earlier data for CO(2) (J = 58-80) to yield the full state-resolved distribution of scattered CO(2) (00(0)0). The scattered CO(2) (00(0)0, J = 2-80) molecules have a biexponential rotational distribution with T(lowJ) = 437 +/- 50 K for J < 50 and T(highJ) = 1145 +/- 110 K for J > 50. Fitting the data with a two-component exponential model yields CO(2) product distributions with T(rot) = 329 and 1241 K. The cooler distribution accounts for 78% of the scattered population and results from elastic or weakly inelastic collisions that induce very little rotational excitation in CO(2). The hotter distribution involves large changes in CO(2) rotational energy and accounts for 22% of collisions. The relative translational energy of the scattered molecules increases as a function of final CO(2) rotational state with T(rel) = 860 K for J = 2, 1650 K for J = 60, and 5500 K for J = 80. The total rate constant for appearance of scattered CO(2) (00(0)0) is k(app) = (4.8 +/- 1.4) x 10(-10) cm(3) molecule(-1) s(-1), which is approximately 85% of the Lennard-Jones collision rate. Population depletion measurements yield a collision rate of k(dep) = (5.7 +/- 1.8) x 10(-10) cm(3) molecule(-1) s(-1), which is in very good agreement with the Lennard-Jones collision rate. The full energy transfer probability distribution function P(DeltaE) for pyrazine(E)-CO(2) collisions is presented and compared to results from other studies.

Effects of Alkylation on Deviations from Lennard−Jones Collision Rates for Highly Excited Aromatic Molecules: Collisions of Methylated Pyridines with HOD

Collision rates and energy transfer distributions are reported for HOD with highly vibrationally excited 2-methylpyridine (2-picoline, E = 38 310 cm(-1)) and 2,6-dimethylpyridine (2,6-lutidine, E = 38 700 cm(-1)). High resolution transient IR absorption is used measured to complete product state distributions of scattered HOD(000) molecules with E(rot) = 109 to 1331 cm(-1). Doppler-broadened line profiles characterize the depletion and appearance for HOD molecules due to collisions with hot donors and show that the product translational and rotational energy distributions are similar for both donors with DeltaE(rel) = 370 cm(-1) and DeltaE(rot) approximately 75 cm(-1). The energy transfer rate for picoline (E)/HOD is 2.5 times larger than the Lennard-Jones collision rate. The energy transfer rate for lutidine(E)/HOD is 3.2 times larger than the Lennard-Jones collision rate. Previous work ( Havey, Liu, Li, Elioff, and Mullin, J. Phys. Chem. A 2007, 111, 13321-9 ) reported similar energy transfer values for pyrazine/HOD collisions and an energy transfer rate that is 1.7 times the Lennard-Jones collision rate. The observed collision rates are discussed in terms of hydrogen bonding interactions between HOD and the aromatic donor molecules. Energy gain profiles for HOD are compared with those for H(2)O.

Dynamics of Weak and Strong Collisions: Highly Vibrationally Excited Pyrazine (E = 37900 cm−1) with DCl

The outcome of energy transfer due to single collisions between highly vibrationally excited pyrazine (E = 37,900 cm(-1)) and DCl is measured for the appearance of individual low-J rotational states of DCl using high-resolution transient IR absorption. Appearance profiles from double-Gaussian transient lineshapes were measured for a number of DCl states with J = 2-12. These data give information on the recoil velocity distributions, appearance rates, and populations of individual states of the scattered molecules. These data complement previous studies on high-J state DCl scattering (Li, Z.; Korobkova, E.; Werner, K.; Shum, L.; Mullin, A. S. J. Chem. Phys. 2005, 123, 174306), and together, they provide a full description of the V-RT collisions with DCl that quench pyrazine(E). Scattered DCl (v = 0) molecules with J = 2-21 are rotationally hot with T(rot) = 880 +/- 100 K. Center-of-mass translational energy distributions are T(rel) approximately 700 K for J < 15. Broader velocity distributions are observed for the J = 15-20 states. The rate constant for V-RT energy transfer is 4.6 x 10(-10) cm(3) molecule(-1) s(-1). This value is a lower limit to the overall rate constant for energy transfer and corresponds to approximately 85% of the Lennard-Jones collision rate. We estimate scattering into the DCl (v = 1) state occurs in approximately 1% of collisions. The energy transfer probability distribution P(DeltaE) is presented and yields DeltaE = 888 cm(-1).

Energy Transfer Dynamics in the Presence of Preferential Hydrogen Bonding: Collisions of Highly Vibrationally Excited Pyridine-h5, -d5, and -f5 with Water

The relaxation of highly vibrationally excited pyridine-h5, pyridine-d5, and pyridine-f5 (E approximately 38,000 cm(-1)) through collisions with water was investigated by high resolution transient IR absorption spectroscopy to investigate how preferential hydrogen bonding interactions impacts the energy transfer dynamics. Nascent rotational and translational energy gain profiles for scattered H2O(000) molecules with E(rot) > 1000 cm(-1) are reported. H2O(000) molecules scattered from pyridine-h5 and pyridine-d5 have rotational distributions with T(rot) = 890 K. Less rotational energy is found in H2O(000) scattered from pyridine-f5 for which T(rot) = 530 K. The recoil energy distributions are similar for the three donors with T(rel) = 400-700 K. To explain the results, a torque-inducing mechanism is proposed that involves directed movement of water between sigma and pi-hydrogen bonding interactions with the pyridine donors. The experimental results are consistent with this mechanism, and with effects due to the state-density energy dependence of the highly excited donor molecules. Differences in vibrational mode frequencies of the hot donor molecules do not appear to explain the experimental results.

Energy-dependent dynamics of large-ΔE collisions: Highly vibrationally excited azulene (E=20390 and 38580cm−1) with CO2

We report the energy dependence of strong collisions of CO(2) with highly vibrationally excited azulene for two initial energies, E=20 390 and 38 580 cm(-1). These studies show that both the distribution of transferred energy and the energy transfer rates are sensitive to the azulene energy. Highly excited azulene was prepared in separate studies by absorption of pulsed excitation at lambda=532 or 266 nm, followed by rapid radiationless decay from S(1) or S(4) to vibrationally excited levels of the ground electronic state. The appearance of scattered CO(2) (00(0)0) molecules with E(rot)>1000 cm(-1) was monitored by high-resolution transient IR absorption at lambda=4.3 mum. The average rotational and translational energies of the scattered CO(2) molecules double when the azulene energy is increased by a factor of 2. The rate of energy transfer in strong collisions increases by nearly a factor of 4 when the azulene energy is doubled. The energy transfer probability distribution function for DeltaE>3000 cm(-1) at each initial energy is an exponential decay with curvature that correlates with the energy dependence of the state density, in excellent agreement with predictions from GRETCHEN, a model based on Fermi's golden rule to describe collisional quenching of highly excited molecules.

Relaxation Dynamics of Highly Vibrationally Excited Picoline Isomers (Evib= 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-Dependent Quantum-State-Resolved Relaxation of Highly Vibrationally Excited Pyridine (Evib = 36 990−40 200 cm-1) through Collisions with CO2

The energy dependence of strong collisional deactivation that transfers energy from highly vibrationally excited pyridine to a CO 2 bath has been investigated for pyridine with internal energy between E v i b = 36 990 and 40 200 cm - 1 . Highly excited pyridine was prepared by absorption of tunable pulsed UV laser light at four wavelengths, λ = 251, 259, 266, and 273 nm, followed by prompt radiationless decay from S 1 (or S 2 ) to the vibrationally excited levels of the ground electronic state. High-resolution transient IR absorption spectroscopy was used to measure the nascent rotational and translational energy distributions of scattered CO 2 molecules in high J states of the ground vibrationless (00 0 0) level that result from collisions with hot pyridine. Our results reveal that substantial amounts of rotational and translational energy are imparted to CO 2 (00 0 0) molecules for all four pyridine energies and that the magnitude of the CO 2 rotational and translational energy gains changes very little for a 3000 cm - 1 change in the vibrational energy of the pyridine donor. State-resolved energy-transfer rate constants were measured at each UV wavelength and were also found to be fairly insensitive to changes in the donor internal energy content for the excitation energies studied here. These results indicate that the relaxation mechanism and the energy-transfer probability distribution function for the V → RT pathway for pyridine(E)/CO 2 collisions are invariant to this change in internal energy. A consideration of energy-transfer rates using Fermi's Golden Rule is consistent with the observed behavior. Comparison with previous energy-dependent collisional quenching studies [Elioff, M. S.: Wall, M.; Lemoff, A.; Mullin, A. S. J. Chem. Phys. 1999, 110, 5578-5588] on vibrationally hot pyrazine with CO 2 suggests that this may be a general phenomenon in the collisional relaxation of highly excited molecules.