Vibrational Predissociation Lifetime Research Articles

Infrared-driven dynamics and scattering mechanisms of NO radicals with propane and butane: impacts of pseudo Jahn-Teller effects.

The topology of multidimensional potential energy surfaces defines the bimolecular collision outcomes of open-shell radicals with molecular partners. Understanding these surfaces is crucial for predicting the inelastic scattering and chemical transformations of increasingly complex radical-molecule collisions. To characterize the inelastic scattering mechanisms of nitric oxide (NO) radicals with large alkanes, we generated the collision complexes comprised of NO with propane or n-butane. The infrared action spectroscopy and infrared-driven dynamics of NO-propane and NO-(n-butane) collision complexes in the CH stretch region were recorded, while also comparing the results to the analogous experiments carried out for NO-CH4 and NO-ethane. The infrared spectroscopy is analyzed using rovibrational simulations to characterize the transition bands and to determine the vibrational predissociation lifetimes of NO-propane and NO-(n-butane). Due to pseudo Jahn-Teller dynamics, the NO-propane and NO-(n-butane) decay mechanisms from IR activation appear similar to those for NO-ethane previously reported from this laboratory (J. P. Davis et al. Faraday Discuss., 2024, 251, 262-278). Furthermore, the NO (X2Π, v'' = 0, J'', Fn, Λ) product state distributions from NO-alkane fragmentation reveal a strong electron-spin polarization and a propensity for NO products to rotate in the plane of the π* molecular orbital, yielding mechanistic insights into the inelastic scattering outcomes. We hypothesize that a geometric phase may be present, impacting the relative population distributions, in addition to the accessible pathway timescales.

Vibrational energy levels and predissociation lifetimes of the A2Σ+ state of SH/SD radicals by photodissociation spectroscopy.

Photo-predissociation of SH and SDradicals in the A2Σ+ state is investigated using the high-n Rydberg atom time-of-flight technique. By measuring the photoproduct translational energy distributions as a function of excitation wavelength, contributions from overlapping A2Σ+ (v') ← X2Π (v″) transitions can be separated, and the H/D + S(3PJ) photofragment yield (PFY) spectra are obtained across various rovibrational levels (SH v' = 0-7 and SDv' = 0-8) of the A2Σ+ ← X2Π bands. The upper A2Σ+ state vibrational levels v' = 5-7 of SH and v' = 3-8 of SDare determined for the first time. The PFY spectra are analyzed with the simulation program PGOPHER [C. M. Western, J. Quant. Spectrosc. Radiat. Transfer 186, 221 (2016)], which gives vibrational origins and linewidths of the rovibrational levels of the A2Σ+ state. The linewidths (≥1.5cm-1) of the SH A2Σ+ v' = 3-7 and SDA2Σ+ v' = 2-8 states are characterized for the first time in this work, demonstrating that these levels undergo rapid predissociation with lifetimes on the order of picosecond. The lifetimes of the SDA2Σ+ v' = 0, N' = 1 and 2 levels are determined to be 247 ± 50ns and 176 ± 60ns by pump-probe delay measurements, respectively. The experimentally measured lifetimes are in reasonable agreement with the theoretical predictions.

The rotationally resolved symmetric 2OH excitation in H2O-CO2 observed using pulsed supersonic expansion and CW-CRDS

The rovibrational band corresponding to a double excitation of the OH symmetric stretch of the H2O unit in the H2O-CO2 van der Waals complex has been recorded using CW-CRDS and a pulsed slit expansion seeded in He. The set-up is presented. The rotational analysis of this band is detailed and the results of the global fit of these data with those of the other 2OH excitation band and of the ground vibrational states are reported including data from the literature. The tunneling frequency and the vibrational predissociation lifetime are shown to decrease with vibrational excitation.

Experimental and theoretical investigations of H2O-Ar.

We have used continuous-wave cavity ring-down spectroscopy to record the spectrum of H2O-Ar in the 2OH excitation range of H2O. 24 sub-bands have been observed. Their rotational structure (Trot = 12 K) is analyzed and the lines are fitted separately for ortho and para species together with microwave and far infrared data from the literature, with a unitless standard deviation σ=0.98 and 1.31, respectively. Their vibrational analysis is supported by a theoretical input based on an intramolecular potential energy surface obtained through ab initio calculations and computation of the rotational energy of sub-states of the complex with the water monomer in excited vibrational states up to the first hexad. For the ground and (010) vibrational states, the theoretical results agree well with experimental energies and rotational constants in the literature. For the excited vibrational states of the first hexad, they guided the assignment of the observed sub-bands. The upper state vibrational predissociation lifetime is estimated to be 3 ns from observed spectral linewidths.

Overtone, 2OH spectroscopy of H2O[sbnd]Kr

We have used continuous-wave cavity ring-down spectroscopy to record the spectrum of H2OKr in the 2OH excitation range of H2O. 11 sub-bands have been observed for the main krypton isotope, 84Kr. Their rotational structure (Trot=18K) is analyzed and the lines fitted together with literature microwave data, with a unitless standard deviation σ=0.86 and 1.32 for ortho and para species, respectively. 4 more sub-bands are observed for the three other isotopes and are also analyzed. The upper state vibrational predissociation lifetime is estimated to 4ns from observed spectral linewidths.

The Water Dimer Investigated in the 2OH Spectral Range Using Cavity Ring-Down Spectroscopy.

Two setups based on CW cavity ring-down spectroscopy were used at Bruxelles and Rennes to record jet-cooled water dimer absorption between 7188 and 7285, and between 7357 and 7386 cm(-1). Some 19 absorption features are reported, significantly more than in the literature. Limited high-resolution information is available due to strong overlap between neighboring vibration-rotation-tunneling (VRT) structures and to spectral broadening induced by short upper state vibrational predissociation lifetimes, likely to range between 100 and 20 ps. Rotational band contours analyses are performed to assign the partly resolved VRT structures to the v1v2v3,vfvb = 000,11; 200,00; 000,20; and 101,00 zero-order vibrational states. Their wavenumbers are found to be 7192.34, 7225.86, 7240.57, and 7256.99 cm(-1), respectively. Both so-called acceptor-switching tunneling components are involved in the assignments whose tentative character is discussed.

Overtone, 2NH (ν1+ν3) spectroscopy of NH3–Ar and NH3–Kr

We have recorded between 6561 and 6671 cm–1 the spectrum of jet-cooled ammonia seeded in Ne, Ar and Kr, using continuous wave cavity ring-down spectroscopy (CW-CRDS). The equivalent absorption pathlength was around 750 m. Three bands are assigned to Π(11)←Σ(00), Π(11)←Π(10) and Δ(?)←Π(10) in the 2NH, ν1+ν3←GS transition of the ortho NH3–Ar dimer. They are rotationally analysed and a simultaneous fitting procedure, together with one far-infrared ground state band from the literature is successfully achieved. A possible Mj-dependent cooling process is reported. Only the first of these bands is observed in the ortho NH3–Kr dimer, and rotationally analysed. Individual line perturbations and anomalous line broadening effects are reported. A J-dependent vibrational predissociation lifetime with a mean value around 0.6 ns is obtained for the Π(11) sub-state in NH3–Ar and NH3–Kr. Two additional bands are assigned to NH3–Ar involving close Π upper sub-states. A group of close bands from the para dimer is identified in NH3–Ar. The energy of all observed ortho and para sub-states is extracted from the analysis. Finally, more bands are reported but their carriers could not be identified.

Effect of the Intermolecular Excitation in the Vibrational Predissociation Dynamics of van der Waals Complexes and the Implications for Control

The effect of intermolecular excitation on the vibrational predissociation lifetime is investigated systematically for different vdW complexes Rg-X2(B, v') (Rg = rare gas atom, X = halogen atom) by means of wave packet simulations. The lifetime as a function of intermolecular excitation displays a pattern of maxima and minima, with a similar shape for the different Rg-X2(B, v') complexes. The pattern is consistent with previous experimental findings involving lifetimes of intermolecular excitations in similar systems. The structure of the lifetime pattern is found to be determined by the shape of the resonance wave functions in the two van der Waals degrees of freedom, and more specifically by the magnitude of the overlap between the wave function and the coupling responsible for predissociation. Lifetime maxima and minima are associated with minima and maxima of this overlap, respectively. Implications for control of the complex lifetime are discussed.

An empirical potential-energy surface for the He–I2(BΠu3) van der Waals complex including three-body effects

An empirical intermolecular potential surface is proposed for the He-I2(B 3IIu) complex, modeled as a sum of pairwise He-I Morse interactions plus a three-body interaction term. The potential reproduces with very good agreement the spectral blueshifts and vibrational predissociation lifetimes measured for He-I2(B,v') in the range v' = 10-67 of I2 vibrational excitations. In particular, the accuracy achieved in the description of the experimental data for high v' levels is attributed to the three-body interaction term included in the potential. The behavior of the potential surface with the I-I separation is analyzed and correlated with the experimental findings.

The vibrational structure and predissociation lifetimes of I 2( B)–Ne: VSCF–DWB–IOS approximation

Recently new experimental findings on vibrational predissociation dynamics of I 2( B)–Ne have been reported. VSCF–DWB–IOS approximate method for vibrational predissociation is applied to understand the predissociation dynamics. Using the vibrational self-consistent field (VSCF) method with a modified potential function, we determined the vibrational structure of I 2( B)–Ne that agrees with experiments very well. Using the distorted-wave Born (DWB) and the infinite-order sudden (IOS) approximations for dissociation process, we have calculated the predissociation lifetimes that are also in good agreement with other theories or experiments. The advantage and limit of the proposed method and the modified potential function are discussed.

OD–N 2 : Infrared spectroscopy, potential anisotropy, and predissociation dynamics from infared-ultraviolet double resonance studies

The infrared spectrum of the linear OD–N2 complex has been recorded in the OD overtone region near 1.9 μm using an infrared-ultraviolet double resonance technique. The pure overtone band of OD–N2(2νOD) was observed at 5173.99(1) cm−1, and combination bands involving the simultaneous excitation of OD stretch and geared bend were identified at 5209.02(1) cm−1 and 5214.59(2) cm−1. Assignments and spectroscopic constants have been derived from the rotationally resolved structure of each band, which are in good accord with model calculations based on an electrostatic interaction potential. Direct time–domain measurements yielded a vibrational predissociation lifetime of 150±16 ns for OD–N2 (2νOD) and a three fold decrease in lifetime upon intermolecular excitation of the lower-energy geared bending state. The OD (v=1) fragments of vibrational predissociation were found to be highly rotationally excited, indicating that predissociation proceeds by vibrational to rotational/translational energy transfer. The results obtained for OD–N2 are compared with an analogous study of OH–N2 [Marshall et al., J. Chem. Phys. 114, 7001 (2001)], revealing insights on the potential anisotropy along the geared bending coordinate and a change in the vibrational predissociation mechanism.

Vibrational predissociation of ArHF: a test of global semiempirical potential energy surfaces

Vibrational spectra and vibrational predissociation dynamics of the ArHF complex in the lowest van der Waals levels corresponding to the vibrational states of the HF fragment v = 0–7 are investigated using an accurate quantum mechanical method and recent three-dimensional potential energy surfaces (PES). The results demonstrate the accuracy of the semiempirical diatomics-in-molecule and empirically corrected ab initio PES's and allow for better understanding of the sensitivity of the vibrational predissociation lifetimes and product state distributions to the interaction potential. Main features of the fragmentation dynamics such as near-resonant vibration-to-rotation energy transfer, strong variation of the predissociation lifetimes with excitation of the van der Waals vibrational modes, and rapid increase of the decay rates with vibrational excitation of HF are analyzed.

Mode-selective decay dynamics of the ortho-H2—OH complex: experiment and theory

The predissociation dynamics of ortho-H2—OH in its ground electronic state are studied in time-resolved experiments and quantum mechanical calculations. The experiments use stimulated Raman and infrared overtone pumping to excite the νH2 = 1 or νOH = 2 vibrational states of the complex. The appearance of the OH X 2Π products is monitored as a function of time. Vibrational predissociation lifetimes and rotational product distributions are calculated in full dimensionality with a time-dependent wavepacket technique using a potential energy surface derived from high quality ab initio calculations. Both experiment and theory give predissociation lifetimes for νH2 = 1 that are an order of magnitude shorter than those for νOH = 2, and only a small number of OH rotational product states are found to be populated. The possibility of inducing chemical reaction via initial vibrational excitation of H2—OH is discussed.

High-resolution diode laser study of H2–H2O van der Waals complexes: H2O as proton acceptor and the role of large amplitude motion

Three high-resolution rovibrational bands of ortho H2 with both para and ortho H2O are observed in a slit supersonic expansion, based on direct absorption of a tunable diode laser in the ν2 bend region of H2O near 1600 cm−1. Complexes containing para H2O are responsible for a Σ←Σ type band associated with intramolecular bending excitation of H2O, while complexes containing ortho H2O exhibit two bands associated with (i) the intramolecular HOH bend (Π←Π) and (ii) an inter+intramolecular combination band (Σ←Π) corresponding to simultaneous HOH bend plus internal rotor excitation. From high-resolution line broadening studies, each upper state has a different vibrational predissociation lifetime; for bend excited para H2O complexes it is 5.1(14) nsec, while for the bend excited state and bend+internal rotor combination state of ortho H2O, it is 2.53(14) and 1.85(33) nsec, respectively. Analysis of the spectra supplemented by 2D quantum calculations indicate large amplitude, slightly hindered internal rotation of the H2O subunit in the complex. Nevertheless, the internal rotor splittings yield potential parameters that suggest ortho H2–H2O is best described with the H2 predominantly pointing towards the O atom in a H2O proton acceptor geometry.

Feshbach resonances in ultracold atom-diatom scattering

Quantum-mechanical scattering calculations of Feshbach resonances arising from van der Waals molecule formation are used to determine vibrational and rotational predissociation lifetimes. A multichannel effective range theory is used to establish the relationship between predissociation and the zero-temperature limit of collisional quenching for resonances lying close to the thresholds for dissociation. The elastic scattering Feshbach resonances may be measurable in ultracold atom-molecule experiments.

Vibrational predissociation of D2HF and H2HF with a new potential energy surface

The vibrational predissociation of D2HF and H2HF is calculated using a time-dependent method. The calculations use an improved potential energy surface obtained from a fit of new ab initio points to a spherical harmonic expansion. The vibrational predissociation lifetimes and rotational product distributions for D2HF agree well with experiment. For H2HF the vibrational predissociation lifetimes are longer than expected from experiment.

Vibrational predissociation of D2HF and H2HF with a new potential energy surface

The vibrational predissociation of D2HF and H2HF is calculated using a time-dependent method. The calculations use an improved potential energy surface obtained from a fit of new ab initio points to a spherical harmonic expansion. The vibrational predissociation lifetimes and rotational product distributions for D2HF agree well with experiment. For H2HF the vibrational predissociation lifetimes are longer than expected from experiment.

Near-infrared laser spectroscopy of the Ar–C 2HD complex: : Fermi resonance assisted vibrational predissociation

The rotationally resolved infrared spectrum is reported for the C–H stretching vibration of Ar–HCCD. A bending combination band is also observed from which we obtain an accurate value for the low-frequency intermolecular bending frequency. The vibrational predissociation lifetime of Ar–HCCD is found to be considerably longer than that of Ar–C 2H 2. This difference is attributed to the strong Fermi resonance in Ar–C 2H 2 which is absence in Ar–HCCD. We propose that the perturbing level in the Ar–C 2H 2 complex provides a `doorway' that accelerates the dissociation in this system. In Ar–HCCD, where this `doorway state' is not Fermi mixed, the dissociation is slower.

The vibrational predissociation of HeBr 2: a wavepacket study

We present a theoretical study of the vibrational predissociation of the Van der Waals complex HeBr 2, using a time-dependent quantum mechanical approach with a symplectic integrator propagator, for total angular momentum J = 0. This method gives not only vibrational predissociation lifetimes, resonance energies and rotational product distributions but also information on the dynamics of the system. A comparison is made with time-independent and experimental results, for initial Br 2 excitation level v=8, 10, 12, and good agreement is found.

Ab initio calculations of the interaction of He with the B 3Π0u+ state of Cl2 as a function of the Cl2 internuclear separation

Ab initio calculations using unrestricted Mo/ller–Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B 3Π0u+ state of Cl2. Also, more reliable unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl–Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B←X, 8←0 spectrum is in good agreement with the experimental one, and the calculated blueshifts for ν=8, 10, 12 show the right trend when compared to the experimental findings, i.e., the blueshifts get larger with increasing ν. The blueshift values are generally too small which suggests that the ab initio calculations underestimate the van der Waals interactions in the B state less than they do in the X state. The calculated vibrational predissociation lifetimes τ are in good agreement with the experiment, as are the rotational product distributions for ν=8, 10, and 12.