Pier Remigio Salvi

Fermi resonance in solid CO2

A theory of Fermi resonance in molecular crystals is described and applied to the ω1; 2ω2 Fermi resonance region of the CO2 crystal. It is shown by means of the Green functions method that bound states can originate out of the two‐phonon manifold of ω2+ω2 vibrational states for sufficiently high values of the anharmonicity constant. Starting from the crystal harmonic Hamiltonian, Green functions for the excitations of interest are built. Introducing as anharmonic contribution to the Hamiltonian the intramolecular coupling between ω1 and 2ω2, renormalization of the ω2 density of states results. The profiles of the renormalized density of states are seen to change both with mechanical anharmonicity and the unperturbed ω1 frequency. Two cases of practical interest are discussed: in the first the ω1 state is immersed into the ω2+ω2 manifold while in the second ω1 is well separated from that. The intensity of the Raman spectrum is discussed in terms of a direct mechanism, i.e. scattering of light by ω2(k) and ...

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication.

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).

On the vibrational assignment of fullerene C60

A recent density functional perturbation theory calculation of the vibrational frequencies of C60 is compared with the infrared spectrum of the crystal. The vibrational assignment of C60 is completed with the help of the calculation plus the available infrared, Raman, and inelastic neutron scattering spectra.

Spectroscopy of some ices of astrophysical interest: SO2, N2 and N2: CH4 mixtures

Abstract Near infrared spectroscopic observations of icy surfaces provide powerful keys to identify specific molecules, and to derive information about the physical and chemical states of the surface ices. In particular, the high spectral resolution recently achievable in astronomical spectra, opens a new insight, but also implies that the complete analysis of these spectra requires careful spectroscopic studies, i.e. clean and systematic laboratory experiments associated with a rigorous interpretation of the spectra. Spectroscopic interpretation is focused on, taking into consideration the specific physical aspects of some ices (molecular solids). It is shown how this analysis allows specific astrophysical problems to be solved. At first, some relevant fundamentals of physics and spectroscopy of molecular solids are presented. The spectroscopy of these solids largely belongs to molecular physics, but also involves solid state effects (Davydov splitting, LO-TO splitting, etc.) which have to be considered to correctly assign spectra, as well as to understand the behaviour of the spectral profile of the bands as a function of various physical parameters. The specific treatment needed to explain the structure of combination and overtone bands occurring in the whole infrared range is particularly focused on. Those theoretical considerations are applied to two different problems concerning surface ices: the first one deals with the identification of two narrow SO2 bands on Io (Schmitt et al., Icarus 111, 79–105, 1994), the second one with the physical state of N2 ice on Triton and Pluto. In a second step, the first results are presented of a systematic spectroscopic study in the near infrared on the two-phase system N2: CH4 for CH4 concentrations ranging from 0.1 to 10%. This study was initiated with the view of investigating the question of the physical state of the surface of Pluto. It is shown that it is possible to investigate the N2: CH4 phase diagram using the spectral profile of both the ν1 + ν4 and the ν3 + ν4 bands of CH4. Finally, the physical parameters (temperature, crystalline phase, etc.) that are expected to be extracted from a detailed analysis of near infrared observations of icy planetary surfaces are briefly reviewed.

High pressure crystal phases of solid CH4 probed by Fourier transform infrared spectroscopy

High pressure infrared spectra of solid CH4 are reported in the range 0.8–30 GPa at room temperature, coupling a Fourier transform infrared spectrometer to a membrane diamond–anvil cell by means of a high efficiency beam condensing optical system. Two crystal phases, A and B, have been investigated. The phase transition is affected by hysteresis and occurs at 9±0.5 GPa during compression and at 7±0.5 GPa during expansion. Due to hysteresis, the transition has been studied as a function of time at higher pressures and found to undergo a first‐order kinetics, with rate constant increasing with pressure. Since our experimental apparatus allows us to perform high pressure Raman measurements too, structural properties of both A and B phases have been proposed from the analysis of the infrared and Raman data. Within the framework of the widely used three‐site model, the A phase structure is consistent with a D4h unit cell symmetry. On the contrary, the analysis of the ω1 infrared and Raman multiplets in phase B...

The two-photon excitation spectrum of pyrene

Abstract The two-photon excitation spectrum of pyrene and the polarization ratio Ω have been measured up to ≈2100 A. Evidence of a strong vibronic couplin

Following Ligand Migration Pathways from Picoseconds to Milliseconds in Type II Truncated Hemoglobin from Thermobifida fusca

CO recombination kinetics has been investigated in the type II truncated hemoglobin from Thermobifida fusca (Tf-trHb) over more than 10 time decades (from 1 ps to ∼100 ms) by combining femtosecond transient absorption, nanosecond laser flash photolysis and optoacoustic spectroscopy. Photolysis is followed by a rapid geminate recombination with a time constant of ∼2 ns representing almost 60% of the overall reaction. An additional, small amplitude geminate recombination was identified at ∼100 ns. Finally, CO pressure dependent measurements brought out the presence of two transient species in the second order rebinding phase, with time constants ranging from ∼3 to ∼100 ms. The available experimental evidence suggests that the two transients are due to the presence of two conformations which do not interconvert within the time frame of the experiment. Computational studies revealed that the plasticity of protein structure is able to define a branched pathway connecting the ligand binding site and the solvent. This allowed to build a kinetic model capable of describing the complete time course of the CO rebinding kinetics to Tf-trHb.

Photoacoustic excitation profiles of gold nanoparticles.

The wavelength dependence of the laser-induced photoacoustic signal amplitude has been measured for water dispersions of 10, 61, and 93 nm diameter gold nanospheres. The whole region of the localized surface plasmon resonance has been covered. This “photoacoustic excitation profile” can be overlayed with the extinction spectrum between 450 nm and 600 nm in the case of the smallest nanoparticles. At variance, the larger-sized nanoparticles display a progressive deviation from the extinction spectrum at longer wavelength, where the photoacoustic signal becomes relatively smaller. Considering that photoacoustics is intrinsically insensitive to light scattering, at least for optically thin samples, the results are in agreement with previous theoretical work predicting (i) an increasing contribution of scattering to extinction when the nanoparticle size increases and (ii) a larger scattering component at longer wavelengths. Therefore, the method has a general validity and can be applied to selectively determine light absorption by plasmonic systems.

On the nature of the two low‐lying singlet states of anthracene: Vibronic activity in the two‐photon excitation spectrum

The two‐photon excitation spectrum of anthracene in a n‐heptane matrix at 10 K has been measured in the energy region 26 000–32 000 cm−1. Experimental evidence of two‐photon band assignment to vibronically induced B1u ×b1u and B2u ×b2u transitions is given. In particular, the two‐photon spectrum above ≂28 000 cm−1 shows several vibronic origins built on b2u vibrations and progressions of ag modes on them. The lowest ππ* absorption region (<28 000 cm−1) has, on the contrary, negligible intensity and very weak B1u ×b1u bands are observed. These data can be rationalized in terms of vibronic coupling between electronic states induced by normalcoordinates of b1u and b2u symmetry. By means of the Herzberg–Teller theory and displacing the molecule away from equilibrium along the normal coordinate we have calculated the vibronic interaction between electronic states in the orbital following approach. The results based on CNDO/S‐CI wave functions show that B2u ×b2u transitions have a larger vibronic activity than ...

Vibrational frequencies of C70

Abstract A vibrational potential function is proposed for C 70 in terms of stretching, bending and non-bonded interactions. The calculated frequencies fit the experimental infrared and Raman active vibrations. The force field is discussed and differences and similarities with previous normal mode calculations are pointed out.