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The process of water evaporation, although deeply studied, does not enjoy a kinetic description that captures known physics and can be integrated with other detailed processes such as drying of catalytic membranes embedded in vapor-fed devices and chemical reactions in aerosol whose volumes are changing dynamically. In this work, we present a simple, three-step kinetic model for water evaporation that is based on theory and validated by using well-established thermodynamic models of droplet size as a function of time, temperature, and relative humidity as well as data from time-resolved measurements of evaporating droplet size. The kinetic mechanism for evaporation is a combination of two limiting processes occurring in the highly dynamic liquid-vapor interfacial region: direct first order desorption of a single water molecule and desorption resulting from a local fluctuation, described using third order kinetics. The model reproduces data over a range of relative humidities and temperatures only if the interface that separates bulk water from gas phase water has a finite width, consistent with previous experimental and theoretical studies. The influence of droplet cooling during rapid evaporation on the kinetics is discussed; discrepancies between the various models point to the need for additional experimental data to identify their origin.

Aerosols are found in a wide diversity of contexts and applications, including the atmosphere, pharmaceutics, and industry. Aerosols are dispersions of particles in a gas, and the coupling of the two phases results in highly dynamic systems where chemical and physical properties like size, composition, phase, and refractive index change rapidly in response to environmental perturbations. Aerosol particles span a wide range of sizes from 1 nm to tens of micrometres or from small molecular clusters that may more closely resemble gas phase molecules to large particles that can have similar qualities to bulk materials. However, even large particles with finite volumes exhibit distinct properties from the bulk condensed phase, due in part to their higher surface-to-volume ratio and their ability to easily access supersaturated solute states inaccessible in the bulk. Aerosols represent a major challenge for study because of the facile coupling between the particle and gas, the small amounts of sample available for analysis, and the sheer breadth of operative processes. Time scales of aerosol processes can be as short as nanoseconds or as long as years. Despite their very different impacts and applications, fundamental chemical physics processes serve as a common theme that underpins our understanding of aerosols. This perspective article discusses challenges in the study of aerosols and highlights recent chemical physics advancements that have enabled improved understanding of these complex systems.

The reaction of CH2ICF3 and other fluorinated or chlorinated iodoalkanes with O(3P), generated by microwave discharge of O2 or 193 nm photolysis of SO2, produces vibrationally excited HF(v) or HCl(v), as observed by steady state or time-resolved Fourier transform infrared (FTIR) emission spectroscopy. This process occurs even in competition with possible pathways to form HOI or IO products. The proposed mechanism is an addition–insertion–elimination process. The nascent vibrational distribution of the HF(v) produced from O+CH2ICF3 is determined to be 0.58±0.10, 0.29±0.08, and 0.12±0.03 for v=1, 2, and 3, respectively, with an upper bound of 0.04 from a few observed lines of v=4. The monotonically decreasing vibrational distribution suggests a reaction involving HF(v) elimination from an intermediate complex. There are a number of possible single or multistep kinetic pathways that could produce HF(v) under these conditions. To determine the predominant pathway that produces the observed HF(v), the dependen...

A new product channel that yields vibrationally excited CO(v=1–9) in the reaction of the ethyl radical with O(3P) is experimentally observed by time-resolved Fourier transform infrared emission spectroscopy. The branching ratios for the different vibrational states are estimated to be 0.21±0.06, 0.27±0.03, 0.14±0.02, 0.08±0.02, 0.07±0.02, 0.07±0.02, 0.06±0.02, 0.05±0.02, and 0.05±0.02 for v=1–9, respectively. Previously, only the CH3+H2CO, CH3CHO+H, and C2H4+OH channels were known. Kinetics tests are provided to verify that the CO is produced directly in the reaction and not from secondary chemistry. The two possible new product channels are CO+CH4+H and CO+CH3+H2. The implications of this previously unexplored reaction channel for combustion chemistry and the possible mechanisms for this reaction are discussed.

The spectral cross-correlation method [Jacobson et al., J. Chem. Phys. 107, 8349 (1997)], developed for the identification and extraction of spectroscopic patterns, is extended to the analysis of product-state dynamical data from photofragmentation. Fragment product state vibrational distributions for the photodissociation of ammonia and deuterated ammonia species are extracted. Since chemical isolation of the mixed isotopic parent molecules is prohibited, the photodissociation dynamics of all four parent species (NH3, NH2D, ND2H and ND3) are studied simultaneously at 193.3 nm. The electronic emission spectra from the NH2(A 2A1), ND2(A 2A1), and NHD(A 2A1) fragments are recorded by time-resolved Fourier transform infrared spectroscopy. Spectral signatures for the photodissociation products from each parent species are extracted by the cross-correlation method. The formalism is derived to extend the spectral cross-correlation method to dynamical reactive product state information. The application of the cr...

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(A 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(A) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(A) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(A,υ2′=0):NH2(A,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(A,υ2′=2) population is ∼10% of the total NH2(A) photofragments. Emission from rotational states with N′>Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(A) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(A) predissociative state onto the NH2(A,υ2′,N′,Ka′)+H photofragments.

Rate constants have been measured for vibration to vibration energy transfer from 12C16O(v=1) to the first vibrationally excited state of both 13C16O (Δν=47 cm−1) and 13C18O (Δν=100 cm−1) using the infrared laser fluorescence technique over the temperature range 50 to 270 K in the gas phase and in liquid neon, deuterium and argon solutions. Fluorescence from 12C16O(v=1) was filtered out, and the time-dependence of the fluorescence intensity from the other isotopomer used to determine the rate constants for energy transfer. The results for the two isotopomeric systems are markedly different. For that with the smaller energy mismatch, 12C16O–13C16O, the rate constants increase with decrease in temperature from 270 to 80 K. This is shown to be consistent with near-resonant energy transfer mediated by transition dipole-transition dipole couplings. Below 80 K, the temperature dependence of the rate constants flattens. For the 12C16O–13C18O system, the rate constants decrease with decrease in temperature from...

Vibrational relaxation cross-sections and rate constants have been calculated for the deactivation of CO(v=1) by 3He and 4He on a new intermolecular potential with vibrational coordinate dependence [T. G. A. Heijmen, R. Moszynski, P. E. S. Wormer and Ad van der Avoird, J. Chem. Phys. 107, 9921 (1997)]. The new surface is found to resolve the qualitative discrepancy between theory and experiment which existed in earlier theoretical calculations. The low impact energy regime has also been investigated focussing in particular on impact energies of less than 15 cm−1 above the vibrational (v=1) threshold. Resonance structure has been found to occur and a comparison is made with an earlier investigation of the low temperature region.

A new N2–He intermolecular potential with vibrational coordinate dependence is presented. Rate constants for the vibrational deactivation of N2(v=1) by He in the gas phase have been calculated over the temperature range 5–300 K. Accurate values of the rate constants for this process are known down to 100 K. We have now extended these measurements down to 70 K for the deactivation of 14N2(v=1) by 4He and down to 50 K for the deactivation of 15N2(v=1) by 3He. Agreement between the theoretically calculated and the experimentally determined rate constants is excellent with the calculated values reproducing the experimental measurements within their error bars. An investigation of the low impact energy regime is also presented. While this focuses on collision energies of less than 20 cm−1 and yields rate constants which are in a temperature region inaccessible to our experimental method, it gives further insights into the influence of the attractive well on vibrational energy transfer processes.

Calculations of the relaxation rate constants, kCO–H2, for the vibrational deactivation of CO(v=1) by pH2 and oH2 are reported in the temperature range 30 K 1 for this system at temperatures above 80 K. Evidence is presented to suggest that below this temperature, which represents the current lower limit of existing experimental data for the CO(v=1)-pH2 system, thermal depopulation of the J=2 rotational state in pH2 reduces the importance of the near-resonant energy transfer process in the determination of kCO–pH2. For T≪80 K the ratio kCO–pH2/kCO–oH2

Calculations of the vibrational relaxation rate constants of the CO–3He and CO–4He systems are extended to lower temperatures than in any previous calculation and a comparison made with new experimental results in the temperature range 35–295 K for CO–3He and previously published results in the range 35–2300 K for CO–4He. Both the coupled states (CS) and infinite‐order sudden (IOS) approximations are used, with the self‐consistent‐field configuration interaction CO–He interaction potential of Diercksen and co‐workers. The CS approximation is found to give a similar level of agreement with experiment for the two isotopic species, while the performance of the IOS approximation is system dependent. The discrepancy between experimental and theoretical IOS rate constants is quite different for collisions involving 3He and 4He, so that it is not profitable to compare IOS results directly with experiment for these two systems at temperatures below 300 K. The differences between the measured and the CS calculated...

Rate constants are presented for (VV) energy transfer between CO(ν=1) and CO2(0001) in the gas phase down to 115 K and in liquid Kr solution at 118 and 130 K. Four isotopically substituted systems were investigated for which the energy mismatches varied between 104 and 306 cm−1. The gas and liquid phase data show several systematic effects with changing energy mismatch. In particular it was found that the ratio of the liquid and gas phase rate constants at the same temperature, kL/kG, increased with decreasing energy mismatch. This is not predicted by current theories of liquid phase energy transfer and is in contrast to previous work using liquid Kr as the solvent.