Time-resolved studies of excited state molecular dynamics

The interaction of light and matter is one that is universal in nature and provides problems to be solved by physicists and chemists alike. This thesis presents a col lection of experiments dedicated to understanding the effect of ultraviolet (UV) and vacuum ultraviolet (VUV) radiation on simple model analogues of biologically and environmentally relevant molecules. The energy deposited into the molecular system through this high-energy radiation is typically redistributed by various nonradiative relaxation processes which take place on ultrafast (femtosecond) timescales. What these processes are and whether they can be related to the molecule’s structure and function will be explored in this text. In order to do this, the highly-differential time-resolved photoelectron imaging (TRPEI) approach was employed in conjunction with theoretical ab initio quantum chemistry calculations. A specific feature that is considered throughout this work is the use of short-wavelength exciting (pump) and ionising (probe) pulses. Initially, acetylacetone was studied using the TRPEI approach in conjunction with 267 nm pump and 160 nm VUV probe pulses. The femtosecond VUV laser pulses were produced using four-wave difference-frequency mixing in an argon-filled gas cell. These high-energy probe pulses provide a significantly extended view along the reaction-coordinate of interest, through a deep projection into the ionisation continuum. This lab-based approach was able to provide quantitative links between elements of earlier reports on relaxation dynamics in acetylacetone, which sampled smaller subsections of the reaction-coordinate. Four dynamical processes occurring on distinct timescales ranging from <10 fs to over 300 ps were identified, including one signature not previously reported. This work highlights the need for such short-wavelength VUV probes in photoionisation-based investigations of photochemical dynamics. Secondly, the non-adiabatic relaxation dynamics of nitrobenzene and three of its dimethyl-derivatives were investigated using TRPEI and ab initio calculations to gain insight into the influence exerted by the nitro-group orientation on the dynamics. Multiphoton ionisation involving two and/or three photons with wave-lengths centred at 400 nm achieved a high-energy probe and revealed near-identical dynamical signatures for all four systems, despite the varying effects of steric hindrance on the nitro-group. These could be assigned to dynamical processes occurring on three timescales: sub 30 fs, in the range of 160-190 fs and finally in the range of 90-160 ps, depending on the molecule. Finally, VUV pulses were again employed, this time as the pump in the study of formamide, N,N-dimethylformamide and N,N-dimethylacetamide, which are motifs ubiquitous in nature. Dynamical signatures indicative of rapid relaxation processes were observed on timescales of 10-35 fs and 70-75 fs in all three systems. In addition to the TRPEI results, extensive quantum chemistry calculations revealed different Rydberg-to-valence evolution behaviour in formamide and the two larger amide systems.Engineering and Physical Sciences Research Council (EPSCR) fundin

Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths 3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility

Decoupling the dark count rate contributions in Ge-on-Si single photon avalanche diodes

Single Photon Avalanche Diodes (SPADs) are semiconductor devices capable of accurately timing the arrival of single photons of light. Previously, we have demonstrated a pseudo-planar Ge-on-Si SPAD that operates in the short-wave infrared, which can be compatible with Si foundry processing. Here, we investigate the pseudo-planar design with simulation and experiment to establish the spatial contributions to the dark-count rate, which will ultimately facilitate optimisation towards operation at temperatures compatible with Peltier cooler technologies

Simulation and Design Optimization of Germanium-on-Silicon Single Photon Avalanche Diodes

Single photon avalanche diodes (SPADs) are semiconductor photodiode detectors capable of detecting individual photons, typically with sub-ns precision timing. We have previously demonstrated novel pseudo-planar germanium-on-silicon SPADs with absorption into the short-wave infrared, which promise lower costs and potentially easier CMOS integration compared to III-V SPADs. Here we have simulated the dark count rate of these devices, using a custom solver for McIntyre’s avalanche model and a trap assisted tunnelling generation model. Calibration and fitting have been performed using experimental data and the results have highlighted areas in which the technology can be optimised

Afterpulsing in Ge-on-Si single-photon avalanche diodes

In this letter, we investigate afterpulsing in 26 and 100 μm diameter planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors, by use of the double detector gating method with a gate width of 50 ns. Ge-on-Si SPADs were found to exhibit a 1% afterpulsing probability at a delay time of 200 μs and temperature of 78 K, and 130 μs at a temperature of 150 K. These delay times were measured with an excess bias of 3.5% applied, which corresponded to a single-photon detection efficiency of 15% at 1.31 μm . We demonstrate that reducing the detector diameter can also be an effective way to restrict afterpulsing in this material system

Surface-normal illuminated pseudo-planar Ge-on-Si avalanche photodiodes with high gain and low noise

Germanium-on-Silicon (Ge-on-Si) avalanche photodiodes (APDs) are of considerable interest as low intensity light detectors for emerging applications. The Ge absorption layer detects light at wavelengths up to ≈ 1600 nm with the Si acting as an avalanche medium, providing high gain with low excess avalanche noise. Such APDs are typically used in waveguide configurations as growing a sufficiently thick Ge absorbing layer is challenging. Here, we report on a new vertically illuminated pseudo-planar Ge-on-Si APD design utilizing a 2 µm thick Ge absorber and a 1.4 µm thick Si multiplication region. At a wavelength of 1550 nm, 50 µm diameter devices show a responsivity of 0.41 A/W at unity gain, a maximum avalanche gain of 101 and an excess noise factor of 3.1 at a gain of 20. This excess noise factor represents a record low noise for all configurations of Ge-on-Si APDs. These APDs can be inexpensively manufactured and have potential integration in silicon photonic platforms allowing use in a variety of applications requiring high-sensitivity detectors at wavelengths around 1550 nm