Laurence Bigio

Two‐photon photodissociation dynamics of state‐selected NO2

Quantum states of NO2 are selected and then photodissociated by resonant two‐photon photoexcitation. The total photolysis energy is scanned over a region from 50 cm−1 below to 300 cm−1 above the threshold for production of NO(X 2Π)+O(1D). This channel yielding excited oxygen is observed to dominate the production of vibrational ground state NO. Diatomic product J and Λ doublet state distributions are probed by resonant two‐photon ionization. The photodissociation cross section for production of specific NO quantum states is found to be structured in the photolysis wavelength. This structure is assigned to intermediate resonance in the two‐photon photolysis. Rotational structure is identified in this pattern, and confirmed by separate optical–UV–double resonance spectroscopy using the same intermediate states in combination with levels of the 3pσ 2Σ+u Rydberg state of NO2. Though photodissociation dynamics are found to be a very sensitive function of photolysis wavelength, different wavelengths that promo...

Intramolecular dynamics and multiresonant absorption spectroscopy. I. Reduced non‐Franck–Condon intensity in the high‐power two‐photon absorption spectrum of NO2

The two‐photon absorption spectrum of the X 2A1→3pσ 2∑+u transition in NO2 shows prominent Franck–Condon forbidden origins. The absolute intensities of these non‐Franck–Condon transitions, as seen by ionization, diminishes at high power. A dense system of dissociative states lies intermediate at the level of the first photon. These states are highly mixed in either bent or linear zeroth‐order basis, and thus at low power effectively bridge bent‐to‐linear origins. At high laser intensity, power (lifetime) broadening prepares a superposition state with the optical character of the ground state and thus poor overlap with low‐lying linear excited states. The preparation and projection of this zeroth‐order, nonstationary intermediate state bears strong analogy to similar absorption and fluorescence processes stimulated by picosecond pulses.

Polarized absorption spectroscopy of Λ‐doublet molecules: Transition moment vs electron density distribution

Polarized two‐photon photodissociation of NO2 in the region of 480 nm yields NO(v‘=0) with an anisotropic distribution of J. Measured polarization ratios are compared to quantum mechanical calculations for a range of expected ratios for the various isolated and mixed branches of the NO X2Π1/2→A 2Σ+ transition. Theoretical results show that main branches and their respective satellites (e.g., R11 and R21 branches) have the same transition moment directionally, though their intensities are in general different, implying that care is needed in interpreting polarization data from the mixed branches, such as (Q21+R11) or (Q11+P21), which measure the Π+ and Π− Λ doublet, respectively. Recognition of this fact is particularly important for properly separating the consideration of electron density distributions of Λ doublets from transition moment directionalities, as this has been a source of confusion in the literature. The measured results indicate that the principal two‐photon photoexcitation pathway in NO2...

High-resolution two-photon spectroscopy of the NO23pσ 2Σu+ Rydberg state

Abstract The high-resolution two-photon absorption spectrum of the X 2 A 1 (000)→3pσ 2 Σ u + (000) system in NO 2 shows rotational fine structure characteristic of a bent-to-linear transition. This structure is well fit by a simple Hamiltonian incorporating an upper state rotational constant B XXX = 0.419 ± 0.001 cm −1 and a spin-rotation coupling constant ψ = 0.150 ± 0.005 cm −1 . These parameters evidence a somewhat contracted Rydberg core NO bond length of 1.126 ± 0.002 A with moderate Rydberg electron-core interaction.

Intramolecular dynamics and multiresonant absorption spectroscopy. II. Power broadening and superposition states in double resonant two‐photon excitation

A simple theoretical model is developed for relative band intensities in multiresonant molecular three‐photon ionization. A specific case is considered in which a dissociative system of states exists at the energy of the first photon and a discreet level system at the second. A third photon ionizes the molecule. Experimentally such a system shows non‐Franck–Condon transitions with relative intensities that depend on laser power. Model calculations trace this dependence to the coherent preparation of a superposition state over the levels at the energy of the first photon that lie within the power‐broadened bandwidth of the laser. For reasonable choice of field and molecule parameters, the model reproduces well the power‐dependent band intensities reported in the previous paper for NO2.

Optical‐selection in the double resonant two‐photon photodissociation of NO2

Optical‐selection is shown to be an effective means to isolate specific initial states for subsequent bound‐continuum photodissociation. States in NO2’s dense visible system are selected and then dissociated near the threshold for production of NO+O(1D) by means of variable wavelength two‐photon photolysis. Product state distributions confirm the excited oxygen pathway as the dominant route for production of NO(v=O). Rotational and Λ‐doublet state populations oscillate in a manner characteristic of the selected intermediate state rotational angular momentum as labeled by separate optical‐optical double resonance absorption spectroscopy.