Development of an experimental set-up for low-temperature spectroscopic studies of matrix-isolated molecules and molecular ices using synchrotron radiation.

An experimental set-up for studying photophysics and photochemistry of molecules in an inert gas medium (matrix-isolated) and in the ice phase at low temperatures has been developed and commissioned at the Photophysics beamline, Indus-1 synchrotron radiation source. This end-station uses an in-house-developed closed-cycle cryostat for achieving cryo-temperatures (∼10 K). Synchrotron radiation from the Photophysics beamline is used as the source of UV-VUV photons and the system is equipped with a Fourier transform infrared spectrometer for characterization of the molecular species formed at low temperature. Various individual components of the end-station like closed-cycle cryostat, experimental chamber, gas mixing and deposition systems are tested to ascertain that the desired performance criteria are satisfied. The performance of the composite system after integration with the Photophysics beamline is evaluated by recording IR and UV-VUV photoabsorption spectra of sulfur dioxide at low temperatures (10 K), both in the ice phase as well as isolated in argon matrices. Results obtained are in good agreement with earlier literature, thus validating the satisfactory performance of the system. As an off-shoot of the study, the VUV absorption spectrum of matrix-isolated SO2 in argon matrix up to 10.2 eV is reported here for the first time. This experimental end-station will provide new opportunities to study photon-induced reactions in molecules of environmental, astrochemical and industrial importance. Details of the design, development and initial experimental results obtained are presented.

Vacuum ultraviolet photoabsorption of prime ice analogues of Pluto and Charon.

Here we present the first Vacuum UltraViolet (VUV) photoabsorption spectra of ice analogues of Pluto and Charon ice mixtures. For Pluto the ice analogue is an icy mixture containing nitrogen (N2), carbon monoxide (CO), methane (CH4) and water (H2O) prepared with a 100:1:1:3 ratio, respectively. Photoabsorption of icy mixtures with and without H2O were recorded and no significant changes in the spectra due to presence of H2O were observed. For Charon a VUV photoabsorption spectra of an ice analogue containing ammonia (NH3) and H2O prepared with a 1:1 ratio was recorded, a spectrum of ammonium hydroxide (NH4OH) was also recorded. These spectra may help to interpret the P-Alice data from New Horizons.

Spectroscopy of N,N-dimethylformamide in the VUV and IR regions: Experimental and computational studies.

The electronic absorption spectrum of N,N-dimethylformamide (DMF) is studied in the 45 000-80 000 cm-1 (5.6-9.9 eV) region using synchrotron radiation. The vacuum ultraviolet (VUV) spectrum comprises mostly of Rydberg series of ns, np, and nd types converging to the first two ionization potentials (IPs). Quantum defect values obtained are consistent with excitation of an electron from the highest occupied molecular orbitals localized on nitrogen (4a″) and oxygen (16a'); in addition, the 3s Rydberg transition converging to the third IP (3a″) is observed at 8.95 eV. A reinvestigation of the infrared spectrum of DMF in the 500-4000 cm-1 region with the help of density functional theory (DFT) calculations establishes the planarity of the ground state and leads to revision of several vibrational assignments. Vertical excited state energies and their valence/Rydberg character are predicted using time dependent DFT calculations; excellent correlation is achieved between theoretical results and experimentally observed spectral features. Potential energy curves of the first few excited states give additional insights into the nature of the excited states and their role in photodissociation dynamics. The absorption spectrum of DMF in the region >63 400 cm-1 (7.85 eV) as well as a complete set of spectral assignments in the VUV region (45 000-80 000 cm-1) is reported for the first time. This work represents a comprehensive study of the absorption spectra of DMF in the VUV and infrared regions.

Qualitative observation of reversible phase change in astrochemical ethanethiol ices using infrared spectroscopy.

Here we report the first evidence for a reversible phase change in an ethanethiol ice prepared under astrochemical conditions. InfraRed (IR) spectroscopy was used to monitor the morphology of the ice using the SH stretching vibration, a characteristic vibration of thiol molecules. The deposited sample was able to switch between amorphous and crystalline phases repeatedly under temperature cycles between 10K and 130K with subsequent loss of molecules in every phase change. Such an effect is dependent upon the original thickness of the ice. Further work on quantitative analysis is to be carried out in due course whereas here we are reporting the first results obtained.

Electronic spectroscopy of ethyl bromide probed by VUV photoabsorption and quantum chemical calculations.

The electronically excited states of ethyl bromide and its deuterated isotopologue (C2H5Br and C2D5Br) are studied using synchrotron radiation based photoabsorption spectroscopy in the wavenumber region 50 000-86 000 cm-1. A detailed spectral analysis supported by quantum chemical calculations is presented. A complex Rydberg series structure comprising of nsa1, npa1, npe, nda1 and nde series, converging to each of the two spin-orbit split components of the first ionization potential (2E3/2 and 2E1/2) is observed for both the isotopologues. Quantum defect values are consistent with excitation from Br lone pair orbitals. Rydberg series analysis is extended to several higher members as compared to earlier work and corroborates the dominance of the spin-orbit mechanism over the hyperconjugative effect. A few new Rydberg series members converging to the second and third ionization potentials are observed and assigned. The complete gas phase VUV photoabsorption spectrum of C2D5Br up to its first ionization limit and its infrared absorption spectrum in the liquid phase are reported for the first time. An extended vibronic analysis of bands accompanying the first few Rydberg series is reported along with several new assignments. DFT calculations on ground states of neutral and ionic species and TDDFT calculations on singlet and triplet excited states aid and support the spectral analysis. Potential energy curves with respect to the C-Br bond length and the C-C-Br bond angle provide further insights into the nature of the excited states. This work represents a comprehensive study of the electronic absorption spectrum of ethyl bromide and its deuterated counterpart.

Ozone and carbon trioxide synthesis by low energy ion implantation onto solid carbon dioxide and implications to astrochemistry.

Ion implantation experiments were carried out on amorphous (30 K) and crystalline (80 K) solid CO2 using both reactive (D(+), H(+)) and non-reactive (He(+)) ions, simulating different irradiation environments on satellite and dust grain surfaces. Such ion irradiation synthesized several new species in the ice including ozone (O3), carbon trioxide (CO3), and carbon monoxide (CO) the main dissociation product of carbon dioxide. The yield of these products was found to be strongly dependent upon the ion used for irradiation and the sample temperature. Ion implantation changes the chemical composition of the ice with recorded infrared spectra clearly showing the coexistence of D3h and C2v isomers of CO3, for the first time, in ion irradiated CO2 ice.

Infrared spectrum of formamide in the solid phase.

Infrared spectra of solid formamide are reported as a function of temperature. Solid formamide samples were prepared at 30 K and then annealed to higher temperatures (300 K) with infrared transmission spectra being recorded over the entire temperature range. The NH(2) vibrations of the formamide molecule were found to be particularly very sensitive to temperature change. The IR spectra revealed a phase change occurring in solid formamide between 155 and 165 K. Spectral changes observed above and below the phase transition may be attributed to a rearrangement between formamide dimers and the formation of polymers is proposed at higher temperatures.

Note: characterization of CaF2/acetone bandpass photon detector with Kr filter gas.

A modified design of a CaF(2)/acetone bandpass photon detector that uses Kr as a filter gas to tune the energy resolution is presented. Our design combines two standard single window detector tubes to build the Kr filter gas chamber. Synchrotron radiation has been used to determine the energy resolution of the detector, as a function of Kr pressure. The improvement in the detector energy resolution by 250 meV compared to the CaF(2)/acetone detector is better than that reported earlier. Substantial variation in the shape of the CaF(2)/acetone detector response functions is observed for different acetone pressure (≤3 mbar), and anode voltage (≤800 V). Our analysis reveals that the changes in the shape of the detector response function are associated to different regions of the detector operation.

VUV electronic state spectroscopy of 1,1-difluoroethene and difluorochloromethane by high resolution synchrotron radiation.

High resolution VUV photoabsorption spectra of 1,1-difluoroethene (C(2)H(2)F(2)) and difluorochloromethane (CF(2)HCl) are reported in the wavelength range 115-200 nm (10.8-6.2 eV). New photoabsorption features are observed in C(2)H(2)F(2) and some Rydberg and vibrational assignments are proposed for the first time. Evidence for a weak vibrational pattern in CF(2)HCl is discussed here for the first time. Absolute cross section values have been obtained allowing photolysis lifetimes to be derived in the Earth's troposphere and stratosphere.