Two-dimensional electronic spectroscopy in the ultraviolet by a birefringent delay line

We introduce a 2D electronic spectroscopy setup in the UV spectral range in the partially collinear pump-probe geometry. The required interferometrically phase-locked few-optical-cycle UV pulse pair is generated by combining a passive birefringent interferometer in the visible and nonlinear phase transfer. This is achieved by sum-frequency generation between the phase-locked visible pulse pair and narrowband infrared pulses. We demonstrate a pair of 16-fs, 330-nm pulses whose delay is interferometrically stable with an accuracy better than λ/450. 2DUV maps of pyrene solution probed in the UV and visible spectral ranges are demonstrated

Fourier transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

We introduce a birefringent interferometer for Fourier transform (FT) spectroscopy in the mid-infrared, covering the vibrational fingerprint region (5-10 μm, 1000-2000 cm-1), which is crucial for molecular identification. Our interferometer employs the crystal calomel (Hg2Cl2), which combines high birefringence (ne-no≈0.55) with a broad transparency range (0.38-20 μm). We adopt a design based on birefringent wedges, which is simple and compact and guarantees excellent delay accuracy and long-term stability. We demonstrate FTIR spectroscopy, with a frequency resolution of 3 cm-1, as well as two-dimensional IR (2DIR) spectroscopy. Our setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy

Conformable nanowire-in-nanofiber hybrids for low-threshold optical gain in the ultraviolet

The miniaturization of diagnostic devices that exploit optical detection schemes requires the design of light-sources combining small size, high performance for effective excitation of chromophores, and mechanical flexibility for easy coupling to components with complex and non-planar shapes. Here, ZnO nanowire-in-fiber hybrids with internal architectural order are introduced, exhibiting a combination of polarized stimulated emission, low propagation losses of light modes, and structural flexibility. Ultrafast transient absorption experiments on the electrospun material show optical gain which gives rise to amplified spontaneous emission, with threshold lower than the value found in films. These systems are highly flexible and can conveniently conform to curved surfaces, which makes them appealing active elements for various device platforms, such as bendable lasers, optical networks and sensors, as well as for application in bioimaging, photo-crosslinking, and optogenetics.Comment: 50 pages, 17 figures, 1 table, ACS Nano, 202

Environment-Driven Coherent Population Transfer Governs the Ultrafast Photophysics of Tryptophan

: By combining UV transient absorption spectroscopy with sub-30-fs temporal resolution and CASPT2/MM calculations, we present a complete description of the primary photoinduced processes in solvated tryptophan. Our results shed new light on the role of the solvent in the relaxation dynamics of tryptophan. We unveil two consecutive coherent population transfer events involving the lowest two singlet excited states: a sub-50-fs nonadiabatic La → Lb transfer through a conical intersection and a subsequent 220 fs reverse Lb → La transfer due to solvent-assisted adiabatic stabilization of the La state. Vibrational fingerprints in the transient absorption spectra provide compelling evidence of a vibronic coherence established between the two excited states from the earliest times after photoexcitation and lasting until the back-transfer to La is complete. The demonstration of response to the environment as a driver of coherent population dynamics among the excited states of tryptophan closes the long debate on its solvent-assisted relaxation mechanisms and extends its application as a local probe of protein dynamics to the ultrafast time scales

A systematic study of the valence electronic structure of cyclo(Gly–Phe), cyclo(Trp–Tyr) and cyclo(Trp–Trp) dipeptides in the gas phase

The electronic energy levels of cyclo(glycine–phenylalanine), cyclo(tryptophan–tyrosine) and cyclo(tryptophan–tryptophan) dipeptides are investigated with a joint experimental and theoretical approach. Experimentally, valence photoelectron spectra in the gas phase are measured using VUV radiation. Theoretically, we first obtain low-energy conformers through an automated conformer–rotamer ensemble sampling scheme based on tight-binding simulations. Then, different first principles computational schemes are considered to simulate the spectra: Hartree–Fock (HF), density functional theory (DFT) within the B3LYP approximation, the quasi-particle GW correction, and the quantumchemistry CCSD method. Theory allows assignment of the main features of the spectra. A discussion on the role of electronic correlation is provided, by comparing computationally cheaper DFT scheme (and GW) results with the accurate CCSD method

Electron and ion spectroscopy of Azobenzene in the valence and core shells

Azobenzene is a prototype and building block of a class of molecules of extreme technological interest as molecularphoto-switches. We present a joint experimental and theoretical study of its response to irradiation with light across theUV to X-ray spectrum. The study of valence and inner shell photo-ionization and excitation processes, combined withmeasurement of valence photoelectron-photoion coincidence (PEPICO) and of mass spectra across the core thresholdsprovides a detailed insight onto the site- and state-selected photo-induced processes. Photo-ionization and excita-tion measurements are interpreted via the multi-configurational restricted active space self-consistent field (RASSCF)method corrected by second order perturbation theory (RASPT2). Using static modelling, we demonstrate that thecarbon and nitrogen K edges of Azobenzene are suitable candidates for exploring its photoinduced dynamics thanks tothe transient signals appearing in background-free regions of the NEXAFS and XP

Exciton Trapping Dynamics in DNA Multimers

Using as a model the single adenine strand (dA)₂₀, we study the ultrafast evolution of electronic excitations in DNA with a time resolution of 30 fs. Our transient absorption spectra in the UV and visible spectral domains show that internal conversion among photogenerated exciton states occurs within 100 fs. Subsequently, the ππ* states acquire progressively charge-transfer character before being completely trapped, within 3 ps, by fully developed charge-transfer states corresponding to transfer of an electron from one adenine moiety to another (A⁺A^–)

Linear absorption spectra of solvated thiouracils resolved at the hybrid RASPT2/MM level

On the example of 2-, 4- and 2,4-thiouracil we demonstrate the performance of the RASPT2/RASSCF protocol in reproducing the spectral positions and line shapes of linear absorption spectra that have been recorded in water and documented in this work. Through a QM/MM scheme coupled to a room-temperature Wigner sampling we simulate condensed phase spectra, permitting to compare our results against experiments. We discuss the sensitivity of the simulations to: a) the active space size by pushing the limits beyond the full-valence active spaces; b) the consideration of “dark” nπ∗-states in the state averaging; c) the flavor of RASPT2 technique; d) the basis set. The benchmarking demonstrates that full-π valence active spaces tend to red-shift the absorption band. Increasing the active space rectifies the problem and we obtain near-quantitative agreement between our experiments and calculations. We, furthermore, demonstrate that the choice of RASPT2 flavor has to be made through rigorous benchmarking

Microjoule-level, tunable sub-10 fs UV pulses by broadband sum-frequency generation

none7siWe introduce a scheme for the generation of tunable few-optical-cycle UV pulses based on sum-frequency generation between a broadband visible pulse and a narrowband pulse ranging from the visible to the near-IR. This configuration generates broadband UV pulses tunable from 0.3 to 0.4 mu m, with energy up to 1.5 mu J. By exploiting nonlinear phase transfer, transform-limited pulse durations are achieved. Full characterization of the UV pulse spectral phase is obtained by two-dimensional spectral shearing interferometry, which is here extended to the UV spectral range. We demonstrate clean 8.4 fs UV pulses. (C) 2014 Optical Society of AmericanoneRocio Borrego Varillas;Alessia Candeo;Daniele Viola;Marco Garavelli;Sandro De Silvestri;Giulio Cerullo;Cristian ManzoniRocio Borrego Varillas;Alessia Candeo;Daniele Viola;Marco Garavelli;Sandro De Silvestri;Giulio Cerullo;Cristian Manzon