Vibrational dynamics of iron pentacarbonyl in cryogenic matrices.

Iron pentacarbonyl is a textbook example of fluxionality. We trap the molecule in cryogenic matrices to study the vibrational dynamics of CO stretching modes involved in the fluxional rearrangement. The infrared spectrum in Ar and N2 is composed of about ten narrow bands in the spectral range of interest, indicating the population of various lattice sites and a lowering of the molecular symmetry in the trapping sites. The vibrational dynamics is explored by means of infrared stimulated photon echoes at the femtosecond scale. Vibrational dephasing and population relaxation times are obtained. The non-linear signals exhibit strong oscillations useful to disentangle the site composition of the absorption spectrum. The population relaxation involves at least two characteristic times. An evolution of the photon echo signals with the waiting time is observed. The behavior of all the signals can be reproduced within a simple model that describes the population relaxation occurring in two steps: relaxation of v = 1 (population time T1 < 100 ps) and return to v = 0 (recovery time > 1 ns). These two steps explain the evolution of the oscillations with the waiting time in the photon echo signals. These results discard fluxional rearrangement on the time scale of hundreds of ps in our samples. Dephasing times are of the same order of magnitude as T1: dephasing processes due to the matrix environment are rather inefficient. The photon echo experiments also reveal that intermolecular resonant vibrational energy transfers between guest molecules occur at the hundreds of ps time scale in concentrated samples (guest/host > 104).

Vibrational spectroscopy and dynamics of W(CO)6 in solid methane as a probe of lattice properties.

Methane solids present more than one accessible crystalline phase at low temperature at zero pressure. We trap W(CO)6 in CH4 and CD4 matrices between 8 and 35 K to probe the interaction between an impurity and its surrounding molecular solid under various physical conditions. Linear and nonlinear vibrational spectroscopies of W(CO)6 highlight different kinds of interaction and reveal new and remarkable signatures of the phase transition of methane. The structures in the absorption band of the antisymmetric CO stretching mode exhibit a clear modification at the transition between phase II and phase I in CH4 and motional narrowing is observed upon temperature increase. The vibrational dynamics of this mode is probed in stimulated photon echo experiments performed with a femtosecond IR laser. A short component around 10 ps is detected in the population relaxation lifetime in the high temperature phase of solid CH4 (phase I) and disappears at lower temperatures (phase II) where the vibrational lifetime is in the hundreds of ps. The analysis of the nonlinear time-resolved results suggests that the short component comes from a fast energy transfer between the vibrational excitation of the guest and the lattice in specific families of sites. Such fast transfers are observed in the case of W(CO)6 trapped in CD4 because of an energy overlap of the excitation of W(CO)6 and a lattice vibron. In solid CH4, even when these V-V transfers are not efficient, pure dephasing processes due to the molecular nature of the host occur: they are temperature dependent without a clear modification at the phase transition.

Stimulated emission in cryogenic samples doped with free-base tetraazaporphine.

Thin cryogenic samples of inert gas solids doped with free-base tetraazaporphine (H2TAP) were irradiated with a tunable pulsed laser. Under resonant electronic excitation of the guest, specific vibronic transitions of the fluorescence spectra were found to be strongly enhanced with only a moderate increase of the laser power. This enhancement is due to stimulated emission (SE). The characteristics of SE bands are described in the three hosts (Ar, N2, and Ne) explored, as well as their excitation spectra. SE is observed in transitions involving different vibrational modes of the guest, depending on the host and the electronic excitation. The results are discussed in comparison with previous works on other tetrapyrrolic molecules trapped in inert gas matrices. From this comparison the key features required to observe SE are deduced to be: (1) SE can be obtained with various tetrapyrrolic molecules; (2) free-base molecules are preferable to their metallo-counterparts; (3) the results highlight a specific molecular vibrational mode involved in the process; and (4) cryogenic crystal structures are also of importance in the detection of SE.

Free base tetraazaporphine isolated in inert gas hosts: matrix influence on its spectroscopic and photochemical properties.

The absorption, fluorescence, and excitation spectra of free base tetraazaporphine (H2TAP) trapped in Ne, N2, and Ar matrices have been recorded at cryogenic temperatures. Normal Raman spectra of H2TAP were recorded in KBr discs and predicted with density functional theory (DFT) using large basis sets calculations. The vibrational frequencies observed in the Raman Spectrum exhibit reasonable agreement with those deduced from the emission spectra, as well as with frequencies predicted from large basis set DFT computations. The upper state vibrational frequencies, obtained from highly resolved, site selected excitation spectra, are consistently lower than the ground state frequencies. This contrasts with the situation in free base phthalocyanine, where the upper state shows little changes in vibrational frequencies and geometry when compared with the ground state. Investigations of the photochemical properties of H2TAP isolated in the three matrices have been performed using the method of persistent spectral hole-burning (PSHB). This technique has been used to reveal sites corresponding to distinct N-H tautomers which were not evident in the absorption spectra. An analysis of the holes and antiholes produced with PSHB in the Qx (0-0) absorption band made it possible to identify inter-conversion of distinct host sites.

Vibrational perturbations of W(CO)6 trapped in a molecular lattice probed by linear and nonlinear spectroscopy.

Vibrational dynamics of the T1u CO stretching mode of tungsten hexacarbonyl is explored when the molecule is embedded in a nitrogen matrix at low temperature. Experiments combined infrared (IR) absorption spectroscopy and IR stimulated photon echoes at the femtosecond time scale. W(CO)6 is found to be trapped in two main families of sites differing by their symmetry (called hereafter Oh and D2h sites). In Oh sites, the vibrational coherence is strongly temperature dependent, exhibiting a coupling with librational phonons of the nitrogen lattice. Perturbation in D2h sites results in the splitting of the T1u band in three components. Each component is inhomogeneously broadened, with dephasing times in the tens of picoseconds, and is weakly coupled to the lattice phonons. Experiments in solid krypton are performed to compare the effect of atomic and diatomic host lattices. Dephasing time in Kr does not depend on temperature and remains in the hundreds of picoseconds, highlighting the molecular origin of the dephasing process in N2. Additionally, nonlinear signals show oscillations due to quantum beats and polarization interferences between different frequency components of the induced third order polarization, giving information, in particular, on the overtone vibrational transition.

Photolithographic processing of silver loaded dielectric coatings based on preformed colloidal TiO2 nanoparticles dispersed in a mesoporous silica binder.

Titanium dioxide is a well known photocatalyst for reactions involving surface trapped photogenerated carriers. Noble metal photo-reduction may be used for the processing of silver/TiO(2) nanocomposite coatings that may exhibit interesting optical and electrical properties. We present here results of our investigations performed on an original system consisting of preformed colloidal TiO(2) nanoparticles homogeneously dispersed within a mesoporous silica host matrix. Light irradiation of samples immerged in an aqueous silver salt solution leads to the homogeneous deposition of silver islands in the vicinity of the TiO(2) particles and throughout the film thickness. The silver volume fraction is directly controlled by the irradiation dose up to a value of about 16 vol.%. Films exhibit tunable plasmonic properties that correspond to silver nanoparticles in interaction, and a percolation threshold is observed at 8-10 vol.%, leading to films with a conductivity of about 40 S cm(-1). The major interest of this method lies in the high silver reduction quantum efficiency (about 50%) and the possibility to modulate optical and electronic properties by light irradiation while the low temperature of processing permits the photolithographic deposition of metallic patterns on organic flexible substrates.

High rotation speed of single molecular microcrystals in an optical trap with elliptically polarized light.

We build an experiment of optical tweezers based on the use of an inverted optical microscope for manipulating microsized single crystals, which are made of an organic dye and parallelepiped in shape. The microcrystals are directed so that their long axis is in the axial direction of the trapping beam. Their short axis follows the direction of the linear polarization of the beam. In circular or elliptic polarization, the crystals are spontaneously put in rotation with a high speed of up to 500 turns per second. It is the first time, to the best of our knowledge, that such a result is reported for particles of the size of our crystals. Another surprising result is that the rotation speed was first increased as expected by increasing the incident power, but after passing by a maximum it decreased until the complete stop of rotation, whereas the power continued growing. This evolution was not reversible. Several hypotheses are discussed to explain such behavior.