RESUMEN
Alignment of OCS, CS_{2}, and I_{2} molecules embedded in helium nanodroplets is measured as a function of time following rotational excitation by a nonresonant, comparatively weak ps laser pulse. The distinct peaks in the power spectra, obtained by Fourier analysis, are used to determine the rotational, B, and centrifugal distortion, D, constants. For OCS, B and D match the values known from IR spectroscopy. For CS_{2} and I_{2}, they are the first experimental results reported. The alignment dynamics calculated from the gas-phase rotational Schrödinger equation, using the experimental in-droplet B and D values, agree in detail with the measurement for all three molecules. The rotational spectroscopy technique for molecules in helium droplets introduced here should apply to a range of molecules and complexes.
RESUMEN
A new technique for obtaining switched wave packets using spectrally truncated chirped laser pulses is demonstrated experimentally and numerically by one-dimensional alignment of both linear and asymmetric top molecules. Using a simple long-pass transmission filter, a pulse with a slow turn-on and a rapid turn-off is produced. The degree of alignment, characterized by ⟨cos2 θ2D⟩, rises along with the pulse intensity and reaches a maximum at the peak of the pulse. After truncation, ⟨cos2 θ2D⟩ drops sharply but exhibits pronounced half and full revivals. The experimental alignment dynamics trace agrees very well with a numerically calculated trace based on the solution of the time-dependent Schrödinger equation. However, the extended periods of field-free alignment of asymmetric tops following pulse truncation reported previously are not reproduced in our work.
RESUMEN
Weakly bound complexes between ferric heme cations and NO were synthesised in the gas phase from ion-molecule reactions, and their absorption measured based on photodissociation yields. The Soret band, which serves as an important marker band for heme-protein spectroscopy, is maximal at 357±5 nm and significantly blue-shifted compared to ferric heme nitrosyl proteins (maxima between 408 and 422 nm). This is in stark contrast to the Q-band absorption where the protein microenvironment is nearly innocent in perturbing the electronic structure of the porphyrin macrocycle. Photodissociation is primarily through loss of NO. In contrast to the Q-band region, two-photon absorption was seen in the Soret band despite NO loss only requiring â¼1 eV. A model based on intersystem crossing to a long-lived triplet state where a barrier has to be surmounted is suggested. Finally, we summarise the measured absorption maxima of heme and its complexes with amino acids and NO.