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.

Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone.

We introduce techniques for probing the dynamics of triplet states. We employ these tools, along with conventional techniques, to develop a detailed understanding of a complex chemical system: a negative-tone, radical photoresist for multiphoton absorption polymerization in which isopropylthioxanthone (ITX) is the photoinitiator. This work reveals that the same color of light used for the 2-photon excitation of ITX, leading to population of the triplet manifold through intersystem crossing, also depletes this triplet population via linear absorption followed by reverse intersystem crossing (RISC). Using spectroscopic tools and kinetic modeling, we identify the reactive triplet state and a non-reactive reservoir triplet state. We present compelling evidence that the deactivation channel involves RISC from an excited triplet state to a highly vibrationally excited level of the electronic ground state. The work described here offers the enticing possibility of understanding, and ultimately controlling, the photochemistry and photophysics of a broad range of triplet processes.

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.

The effect of CO rotation from shaped pulse polarization on reactions that form C2.

The effect of CO rotational energy on bimolecular reactions to form electronically excited C2 is reported here. The reactions are initiated by CO multiphoton absorption of 800 nm light in strong optical fields using two different polarization configurations based on shaped chirped pulses. The observation of Swan band emission indicates that C2(d3Πg) is a reaction product. The optical polarization is in the form of either an optical centrifuge or a dynamic polarization grating. In each case, the strong field aligns CO molecules and induces multiphoton absorption. Power-dependent measurements indicate at least seven photons are absorbed by CO; CO(a3Π) is a likely reactant candidate based on kinetic modeling. Relative reaction efficiencies are determined by measuring Swan band emission intensities. For a CO pressure of 100 Torr and an optical intensity of I = 2.0 × 1013 W cm-2, the relative C2(d3Πg) yield with the dynamic polarization grating is twice that with the optical centrifuge. The extent of CO rotational energy was determined for both optical polarizations using high-resolution transient IR absorption for a number of CO states with J = 62-73 and Erot up to 10 400 cm-1. Optical centrifuge excitation generates at least 2.5 times more rotationally excited CO molecules per quantum state than the dynamic polarization grating. The results indicate that the effect of large amounts of CO rotational energy is to reduce the yield of the C2 products.

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.

Performance of a high-resolution mid-IR optical-parametric-oscillator transient absorption spectrometer.

We report on a mid-IR optical parametric oscillator (OPO)-based high resolution transient absorption spectrometer for state-resolved collisional energy transfer. Transient Doppler-broadened line profiles at λ = 3.3 µm are reported for HCl R7 transitions following gas-phase collisions with vibrationally excited pyrazine. The instrument noise, analyzed as a function of IR wavelength across the absorption line, is as much as 10 times smaller than in diode laser-based measurements. The reduced noise is attributed to larger intensity IR light that has greater intensity stability, which in turn leads to reduced detector noise and better frequency locking for the OPO.

Full state-resolved energy gain profiles of CO2 from collisions with highly vibrationally excited molecules. II. Energy-dependent pyrazine (E = 32,700 and 37,900 cm(-1)) relaxation.

The full state-resolved distribution of scattered CO2 (00(0)0) molecules from collisions with highly vibrationally excited pyrazine (E = 32,700 cm(-1)) is reported and compared to previous studies on pyrazine (E = 37,900 cm(-1)) to investigate how internal energy content impacts the dynamics for collisional quenching of high energy molecules [J. Phys. Chem. A 2010, 113, 1569]. Nascent rotational and translational energy profiles for scattered CO2 (00(0)0) molecules with J = 2-72 were measured using high-resolution transient infrared absorption and combined with earlier results for the J = 56-78 states [J. Chem. Phys. 1999, 111, 7373]. The product translational energy for individual J-states increases by 50% for a 16% increase in donor vibrational energy. The nascent rotational distribution for scattered CO2 is biexponential, comprising 77% nearly elastic collisions and 23% inelastic collisions. The spread of the rotational distribution is sensitive to donor energy, but the branching ratio for elastic and inelastic collisions is the same for both donor energies. The measured collision rates are close to the Lennard-Jones values and are only weakly dependent on changes in donor energy. The nascent energy gain distribution function P(ΔE) depends strongly on the energy, and this energy dependence is stronger than the linear dependence seen in multicollision energy transfer studies for pyrazine(E) + CO2 collisions.

Spectroscopy of molecules in very high rotational states using an optical centrifuge.

We have developed a high power optical centrifuge for measuring the spectroscopy of molecules in extreme rotational states. The optical centrifuge has a pulse energy that is more than 2 orders of magnitude greater than in earlier instruments. The large pulse energy allows us to drive substantial number densities of molecules to extreme rotational states in order to measure new spectroscopic transitions that are not accessible with traditional methods. Here we demonstrate the use of the optical centrifuge for measuring IR transitions of N2O from states that have been inaccessible until now. In these studies, the optical centrifuge drives N2O molecules into states with J ~ 200 and we use high resolution transient IR probing to measure the appearance of population in states with J = 93-99 that result from collisional cooling of the centrifuged molecules. High resolution Doppler broadened line profile measurements yield information about the rotational and translational energy distributions in the optical centrifuge.