Ruchama Fraenkel

Trapping of guests in a rare gas matrix: A molecular dynamics simulation

A molecular dynamics simulation of the trapping of a guest molecule in a rare gas matrix deposition is presented. Using Lennard‐Jones pairwise potentials, the shape and size of trapping sites are seen to depend on the preparation conditions, particularly the temperature and the cooling rate. The proper way to obtain a defect‐free matrix is by depositing at temperatures that are somewhat lower than the annealing temperature.

Molecular dynamics simulations of rare gas matrix deposition

Abstract Molecular dynamics simulations of the trapping of molecules in a rare gas matrix based on a fcc lattice is presented. In distinction with most previous simulations, a sites structure is not obtained by a guess and optimize method, but is found by allowing the lattice to grow by adding atoms from the vapor onto the potential field of a “seed” crystal. Simple pairwise potentials are used to simulate the growth of rare gas matrices containing some small guests, including the diatomic molecules N2, NBr and Br2. Changes in the structure and energy of the solid are calculated, as well as the vibrational frequencies of the trapped molecules, yielding the matrix shift. Novel site geometries, in which the host lattice atoms are displaced from their equilibrium crystal positions, are observed for guest molecules that can strongly distort the lattice because of asymmetry or size mismatches. In these cases, rapid cooling of the incoming atom is essential to the formation of several distinct trapping sites.

The vibrational structure of the S0 → S1 transition of anthracene

Abstract An ab initio calculation of the energy, geometry and vibrational frequencies of anthracene in the ground and first excited singlet state is presented, and compared with recent matrix isolation fluorescence results. It is found that a single configuration can reproduce the experimental data for S1 reasonably well: the 0,0 transitions energy is within 3% of the experimental value, and most observed vibronic bands in S1 can be assigned. The calculation helps to clarify some peculiarities of previous assignments, particularly for modes of the b1g symmetry. Some bands observed in the supersonic jet and in the matrix, and showing anomalously long decay times and unusual matrix shifts are found to fit with the calculated b3g modes. It is tentatively suggested that they are due to Herzberg-Teller vibronic coupling with a B1u+ state, whose presence is indicated in a recent synchrotron fluorescence excitation study.

The ICN-INC system: experiment and quantum chemical calculations

Abstract Photolysis of ICN in an argon matrix leads to the formation of INC, identified by the appearance of three infrared absorption bands. The assignment is confirmed by comparing the vibrational frequencies and absorption band intensities with good quality ab initio calculations. The endothermicity of the isomerization reaction and the ground state barrier to INC formation is also computed. The good agreement between theory and experiment demonstrates the use of quantum chemical calculation as an analytical tool and highlights the partnership of experiment and theory in chemical research.

Photolysis of ICN in a cryogenic matrix

Abstract ICN was deposited at low temperatures in rare-gas matrices and irradiated by UV light at energies ≈ 1.5 eV above its gas-phase dissociation limit. A new infrared band, red-shifted by about 120 cm−1 with respect to the CN stretch band of ICN appeared immediately, and reached its maximum amplitude in 1–2 min. The overall conversion of ICN to products was less than 5%. The new band is tentatively assigned to INC formed by photoisomerization. The low overall yield is due to re-formation of ICN as the nascent INC cools down.

A matrix isolation study of the fluorescence of anthracene and anthracene-ammonia adducts in solid argon

Abstract The UV absorption and emission spectra of anthracene in an argon matrix were measured. Three major spectroscopic systems were observed, assignable to anthracene molecules occupying distinct sites. The origins of these systems were red-shifted with respect to the isolated molecule by 535,691 and 722 cm −1 . At 17 K all lines were well resolved, with about 8 cm −1 fwhm. Addition of a small amount of ammonia leads to the observation of three new band systems, red-shifted with respect to the previously mentioned ones. These observations are discussed in relation to recent studies of anthracene—argon and anthracene—ammonia clusters in supersonic jets.

A molecular dynamics simulation of NBr trapping in an argon matrix

Abstract A molecular dynamics model simulating the preparation of an argon matrix containing a NBr molecule is presented, and compared with recently reported experimental data. The present model reproduces the experimental spectral shift of the NBr vibration in the most stable site reasonably well. The blue-shift of this single substitution site can be related to the shortened NBr equilibrium distance, R e , and to the increased restoring force acting on the nitrogen atom, that finds itself in an interstitial site. Under more stringent cooling conditions, some multiple substitution sites are formed; in these sites, motion is less hindered, the vibrational frequency and R e being similar to those of the free molecules.