We report the cooling of nitric oxide using a single collision between an argon atom and a molecule of NO. We have produced significant numbers (108 to 109 molecules per cubic centimeter per quantum state) of translationally cold NO molecules in a specific quantum state with an upper-limit root mean square laboratory velocity of 15 plus or minus 1 meters per second, corresponding to a 406 plus or minus 23 millikelvin upper limit of temperature, in a crossed molecular beam apparatus. The technique, which relies on a kinematic collapse of the velocity distributions of the molecular beams for the scattering events that produce cold molecules, is general and independent of the energy of the colliding partner.

Rotational state-resolved differential cross sections (DCS) for spin–multiplet-changing collisions of NO(X 2Π1/2→X 2Π3/2) with Ar are presented and compared to results from previous experimental and theoretical studies performed on the NO/Ar system. A crossed molecular beam apparatus coupled with velocity-mapped ion imaging was used to measure complete (θ=0°–180°) DCS for scattering of NO(X 2Π1/2,j=0.5) into NO(X 2Π3/2,j′) rotational states ranging from j′=1.5 to j′=12.5. Scattered products were detected by state-selective ionization using (1+1′) resonance-enhanced multiphoton ionization via the A 2Σ+ state. State-to-state DCS were extracted in the center-of-mass frame of reference for energy transfer at a center-of-mass collision energy of ∼530 cm−1. Studies performed using horizontally and vertically polarized excitation laser beams yielded DCS which were remarkably similar, indicating that state-to-state scattering for this system is insensitive to probe beam polarization. Experimentally determined angular scattering distributions show primarily forward scattering for low-energy rotational states (j<7.5), with side- and back-scattering increasing with product angular momentum. The scattering results are compared and contrasted to results from earlier experimental investigations and to theoretical results from quantum close-coupling calculations based on ab initio coupled cluster CCSD(T) potential energy surfaces.

The quenching of highly vibrationally excited pyrazine through collisions with H2O at 300 K in a low-pressure environment was investigated using high-resolution transient absorption spectroscopy of water at λ ≈ 2.7 μm. Highly vibrationally excited pyrazine with E vib = 37 900 cm-1 was prepared by absorption of 266 nm light to the electronically excited S2 state, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in rotational and translational excitation of the vibrationless ground state of H2O (000) were investigated by measuring the state-resolved appearance of individual rotational states of H2O (000). Transient absorption measurements have been made on numerous rotational states to determine the nascent distribution of rotational energy gain in water. Doppler-broadened transient absorption line shapes were collected for a number of rotational levels in the (000) state in order to measure velocity distributions of the scattered water molecules. The nascent distribution of water rotational states with E rot > 1000 cm-1 is well described by T rot = 920 K, and the velocity distributions correspond to T trans ≈ 560 K, independent of the rotational state. Rate constants for energy gain into individual quantum states of H2O (000) from collisions with hot pyrazine provide a measure of the high-energy part of the energy-transfer probability distribution function. The quenching of hot pyrazine through collisions with water displays a significant reduction in the bath translational energy gain when compared to earlier studies on the quenching of hot pyrazine (E vib = 37 900 cm-1) by CO2 {Wall, M. C.; Mullin, A. S. J. Chem. Phys. 1998, 108, 9658}. A comparison of the two systems provides insights into the molecular properties that influence the relaxation of highly vibrationally excited molecules.

Energy dependent studies of the collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 were performed for initial pyrazine energies Evib=31 000–35 000 cm−1. These studies are presented along with earlier results for pyrazine with Evib=36 000–41 000 cm−1. High-resolution transient IR laser absorption of individual CO2 (0000) rotational states (J=56–80) was used to investigate the magnitude and partitioning of energy gain into CO2 rotation and translation, which comprises the high energy tail of the energy transfer distribution function. Highly vibrationally excited pyrazine was prepared by absorption of pulsed UV light at seven wavelengths in the range λ=281–324 nm, followed by radiationless decay to pyrazine’s ground electronic state. Nascent CO2 (0000) rotational populations were measured for each UV excitation wavelength and distributions of nascent recoil velocities for individual rotational states of CO2 (0000) were obtained from Doppler-broadened transient linewidth measurements. Measurements of energy transfer rate constants at each UV wavelength yield energy-dependent probabilities for collisions involving large ΔE values. These results reveal that the magnitude of large ΔE collisional energy gain in CO2 (0000) is fairly insensitive to the amount of vibrational energy in pyrazine for Evib=31 000–35 000 cm−1. A comparison with earlier studies on pyrazine with Evib=36 000–41 000 cm−1 indicates that the V→RT energy transfer increases both in magnitude and probability for Evib>36 000 cm−1. Implications of incomplete intramolecular vibrational relaxation, electronic state coupling, and isomerization barriers are discussed in light of these results.