Julie Goodman

Hydrogen bonding and charge transfer: Interaction of OH radical with rare gas atoms

Hydroxyl radical in solid Ne at 4.2 K is known to rotate freely, vibrationally relax slowly, and have an essentially unperturbed structure. However, the HO structure and dynamics are profoundly affected by solvation in the heavier rare gases Ar, Kr, and Xe. One Ar atom nearest neighbor increases the OH (A 2Σ+) vibrational relaxation rate by a factor of ?103, and A 2Σ+↔X 2Π spectra consistant with a linear–linear transition in hydrogen bonded ArHO are observed. The Ar–HO (A 2Σ+) well depth is D0?675 cm−1 with ωe?203 cm−1 and ωexe?13.1 in solid Ar host; the Kr–HO (A 2Σ+) well depth is ?1000 cm−1. The Ar–HO hydrogen bond length is shorter by Δre=1.15±0.1 A in the excited state. The OH (A 2Σ+) (0,0) emission band shifts from 3090 in Ne to 4400 A in Xe. The theory of hydrogen bond formation in both ground and excited states is discussed, with particular reference to the contribution of charge transfer in various environments. Comparison is made with the spectra of hydrogen halides, and the physical origin of r...

Structure and energy transfer within isolated (O2)2 dimers via high resolution electronic spectroscopy

Polarized, high resolution 1Δg+1Δg ↔3Σg−+3Σg−(1Ag) and 3Σg−+1Σg+→3Σg−+3Σg− (3B3u) (O2)2 dimer spectra have been observed in solid neon host at 4.2 K. Excited state electronic fine structure components are resolved and assigned. The 1Δg+1Δg transition is both allowed and vibronically induced by b3u antisymmetric O2 stretch quanta. The 1Δg+1Δg geometry is identical to that of the ground 3Σg−+3Σg− state; the 3Σg−+1Σg+ state appears to have slightly different structure. The spectra are entirely consistent with D2h structure, and less likely with D2d structure. The dimer triplet (3B3u) ground state subcomponents lie ∼55 cm−1 above the dimer singlet (1Ag) ground state. The dimer vibrational constants are unperturbed from the O2 in vacuum values. The nearest neighbor vibrational energy transfer time from one O2(1Δg) to another is ∼14 ps, as revealed by zero phonon line splittings in emission. The nearest neighbor electronic energy transfer time of the 1Σg+(v=0) state appears to be 0.6 ps, as revealed by spectral...

Electronic spectroscopy and dynamics of the low‐lying A 3Σ+u, C 3Δu, and c 1Σ−u states of O2 in van der Waals solids

The 2900–2400 A absorption of O2 in solid solutions is dominated by the C 3Δu←X 3Σ−g transition, unlike O2 in the gas phase. Isotopically resolved excitation spectra of v=0 C 3Δ1 emission in solid N2 host provide detailed vibrational and electronic C 3Δ spectroscopic parameters. The data establish absolute vibrational numbering for C 3Δ and c 1Σ, in the gas phase as well as in the solid. A progression of perturbations between v C 3Δ2 and v−2 A 3Σ+u is analyzed to show that this host induced interaction is characterized by a 27 cm−1 matrix element. Vibrational relaxation within both singlet and triplet excited states is ’’fast’’ in N2, Ar, and Xe hosts. It is experimentally proven that these spectra occur in isolated O2 monomers. Excited O2 photochemistry appears to occur in Xe host, but not in N2 host. A double resonance experiment of Rebane et al. is reinterpreted.

Long range vibrational energy transfer from ND and NH(A 3Π) to CO and N2 in solid Ar

Three exothermic, yet near resonant (ΔE=13–126 cm−1) vibrational energy transfer processes from ND (A 3Π) to 12CO and 13CO in solid Ar have been investigated. The transfer quantum yield and the nonexponential time‐resolved donor population both follow Forster (dipole–dipole) kinetics as a function of acceptor concentration. The rates exhibit a strong energy gap (transfer exothermicity) law of the form Kα exp(−ΔE/28 cm−1). Reverse transfer of population from 13CO to ND (A 3Π) is not observed, thus indicating a fast sequential exactly resonant transfer of quanta among CO molecules. CO increases the ND X 3Σ→A 3Π inhomogeneous linewidth; however, the transfer kinetics are independent of excitation wavelength within the inhomogeneous linewidth. Transfer from NH(A 3Π) to 12CO is anomalously fast in view of the large ΔE=728 cm−1. Theory is used to relate the energy transfer rates to the vibrational relaxation rates of the isolated donor and acceptor. The hydride dissipates the exothermicity into lattice phonons ...

Excited state spectroscopy, subpicosecond predissociation, and solvation of diatomic XeO in solid rare gas hosts

Time, wavelength, and polarization resolved fluorescence techniques are used to investigate the photophysics of weakly bound diatomic XeO following frequency doubled dye laser excitation near 2200 A. Photoselection experiments prove that the ’’Green bands’’ contain both E 1Σ→C 1ΣΠ and E 1Σ→B 1Σ fluorescences. The B 1Σ state, which correlates with O(1D)+Xe, is at least three vibrational quanta deeper than previously thought. An isotopic reversal of linewidths between Xe16O and Xe18O in the E 1Σ→B 1Σ spectra is interpreted in terms of subpicosecond homogeneous predissociation of B 1Σ. Solid phase homogeneous predissociation theory is discussed, and the isotopic sensitivity of the appropriate Franck–Condon factors is numerically investigated. The C 1Π and B 1Σ potentials are unambiguously deeper and more harmonic in solid Ar than in vacuum. This change in potential curve shape appears to reflect increased admixture of charge transfer (Xe+O−) components in the solid phase electronic wavefunctions.

Mechanism of vibrational relaxation in molecular solids

The NH and ND(A 3Π) vibrational relaxation rates, and the X 3Σ−?A 3Π spectra, have been investigated in different types of amorphous solid rare gas environments. The vibrational relaxation Hamiltonian is short range, and effectively sees only the nearest neighbors. The relaxation rates are independent of whether the local neighborhood host phonon structure is localized or delocalized. In mixed Ar+Kr hosts, NH* relaxes at the same rate with one or two nearest Kr neighbors as in pure Kr. As the number of Kr nearest neighbors increases, the guest–host interactions, controlling both spectra and relaxation, saturate and are not pairwise additive. This result is qualitatively consistent with gas phase van der Waals complex studies indicating incipient chemical bond formation. The correct local mode theoretical formulation of the solid phase relaxation problem is discussed.

Rydberg states in condensed phases: Evidence for small ’’bubble’’ formation around NO 3sσ (A 2Σ+) in solid rare gases

The lowest Rydberg state of NO has been observed to fluoresce with unity quantum yield in solid Ar, Kr, and Xe hosts. The purely radiative lifetimes lengthen in the sequence vacuum, Ar, Kr, Xe, thus directly demonstrating an increasing delocalization of the Rydberg wavefunction in media with increasing dielectric coefficient. A simple, semiquantitative theory of the relative stability of bubble and Wannier Rydberg states favors bubble (∼10 A diameter) formation around low n Rydberg states in the lighter rare gas solids. Asymmetric phonon contours in absorption and fluoresence, as well as the experimental extent of delocalization compared with Wannier model predictions, both support bubble formation around NO 3sσ (A 2Σ). Sequential two photon excitation spectra from 3sσ to higher Rydberg states near 7.5 eV are broad in all three hosts. The Rydberg 3sσ spectra and photophysics are compared with those of the nearly isoenergetic B 2Π and a 4Π states in both 14NO and 15NO. Rydberg–valence perturbations in the ...

Distant intramolecular interaction between identical chromophores: The n‐π* excited states of p‐benzoquinone

The structure and dynamics of the p‐benzoquinone n‐π* excited states have been investigated in solid Ne host. The two carbonyl excitations are essentially degenerate, with a B1g–Au splitting of +64 cm−1 between the singlets and −11 cm−1 between the triplets. There is no evidence that the lower 3B1g state has a double minimum potential via a pseudo Jahn–Teller distortion. The two triplets have very similar charge distributions and structures, despite their different formal symmetries. Excited singlet vibronic states show evidence for lifetime broadening due to intersystem crossing. Experiments in crystalline hosts are reviewed and reinterpreted. The structure in Ne host is closer to the intrinsic gas phase structure than are the structures in various single crystal environments. Valence bond theory, rather than molecular orbital theory, appears to be the natural description of the electronic structure.

Weak isotope effect in the condensed phase vibrational relaxation of a nonhydride molecule: NO(a4Π)

The 1933 A ArF excimer laser has been used to directly time resolve the vibrational relaxation of 14N16O, 15N16O, and 14N18O into the v=0 B 2Π1/2 and v=0 a 4Π5/2 states in solid Ar host. A weak ’’conventional’’ istope effect is observed, with the smaller vibrational quanta tending to relax faster. However, the rates do not strictly correlate with either the isotopic reduced mass or the asymmetry of vibration. The host lattice phonons may directly accept energy during these relaxation processes. Intersystem crossing between v=0 B 2Π and v=8 a 4Π is shown to involve the individual spin orbit components of a 4Π.

Local mode involvement in the vibrational relaxation of isolated (O2)2 dimers

Study of five isotopic species of isolated (O2)2 dimer in solid neon host near 4.2 °K shows that the mechanism of 1Δg+1Δg excited state vibrational relaxation involves intersystem crossing into the 3Σ−g+1Σ+g state. Several reversible dynamic equilibria are observed between nearly degenerate 1Δg+1Δg and 3Σ−g+1Σ+g vibronic states. Indirect arguments suggest that the nearest neighbor, nonresonant vibrational energy transfer time from 16O2(1Δg) to 18O2(1Δg) is order‐of‐magnitude 10−9–10−10 s. Vibrational relaxation rates for (0+v) 3Σ−g+1Σ+g states (v?3) are slower with heavier reduced masses. The relaxation rate depends not only on the nuclei in the excited O2, but also on the nuclei in the unexcited nearest neighbor O2. These results are consistent with O2 rotation accepting energy during the vibrational relaxation process. This appears to be the first such observation for a nonhydride molecule in condensed phase. The relation between spectral polarization and rotation as the accepting mode is discussed.