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We report on the dynamics of multiphoton excitation and dissociation of NO2 at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm-2. The breakup of the molecule is monitored by NO A 2Σ+n′=1,0→X 2Πrn″=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n′=0 and 1 NO A 2Σ+ results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO2 valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A 2Σ+n′=0→X 2Πrn″=0 fluorescence are consistent with delayed generation of NO A 2Σ+n′=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed à 2B2 and Rydberg 3sσ 2Σg+ states of the parent molecule. The increasingly prompt decay of A 2Σ+n′=1→X 2Πrn″=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X 2A1 and 3sσ 2Σg+ dressed states to generate vibrationally excited products. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The B [SUP3]Σ[SUP-,SUBg] ← X[SUP3]Σ[SUP-,SUBg] transitions of HC[SUB13]H and HC[SUB19]H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] spectra have been observed for HC[SUB2n+1]H (n = 3 - 6) than before with many new vibronic bands identified. The spectra were obtained by means of a mass selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The electronic structures of this series of molecules (n = 2 - 9) in both the ground and excited states have been investigated using DFT, MP2, and state-averaged CASSCF theories. The three lowest dipole allowed electronic transition systems are A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], and C [SUP3]Γ[SUBu] ← X[SUP3]Σ[SUP-,SUBg], located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system is found to be of medium intensity and the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energy transition is not strongly dependent on the length of the chain, but that of the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system increases monotonically with size. The C [SUP3]Γ[SUBu] state is Rydberg in character. The astrophysical implications are considered and an upper limit of the column densities of these carbon chains in diffuse clouds has been estimated as 10[SUP13] cm[SUP-2], based on calculated oscillator strengths. [ABSTRACT FROM AUTHOR]

The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T1. At higher energy (λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol−1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band. [ABSTRACT FROM AUTHOR]

The 3³Πgg state of the dicarbon molecule, C2, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 4³Πgg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 3³Πgg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d³Πgg state. [ABSTRACT FROM AUTHOR]

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionizationenergy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4³Πg←a³Πu band system of C2, the ionizationenergy of the lowest rovibronic level of the a³Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionizationenergy of the forbidden origin of the a³Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionizationenergy of X¹Σg+X¹Σg+ state C2 is 11.866(5) eV. [ABSTRACT FROM AUTHOR]

A high-resolution single vibronic level emission study from the à 2Π3/2 state of the HC4S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes ν2, ν3, and ν5 in the region of 0–3250 cm-1, while the frequency of ν4 remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the à 2Π3/2←X 2Π3/2 laser induced fluorescence spectrum. [ABSTRACT FROM AUTHOR]