Q+A: The Lord Porter Ultrafast Laser Laboratory
by Abbie Sinclair, MChem Chemistry, and James Shipp, PhD student. Originally published in issue seven of Resonance.
Last year, the University of Sheffield's new facility for ultrafast laser spectroscopy was opened by Lady Porter in honour of her late husband George, a Nobel Prize winner and former lecturer in our department. Abbie Sinclair interviewed Professor Julia Weinstein to find out more about why she set up this laboratory, and what it will be used for.
Could you tell us about the new laser lab?
The lab boasts many advanced laser methods, allowing us to study light-induced reactions from tens of femtoseconds, which is close to the speed at which light is absorbed, to milliseconds. We can cover the whole range of energies, from low energy vibrations in the infrared to high-energy electronic transitions in the UV. We can investigate chemical reactions along a huge range of timescales and energies - that’s an interesting combination that doesn’t exist in any other UK university.
Collaborators from all over the UK will be coming and measuring everything related to photovoltages, artificial photosynthesis, fundamental light-matter interactions and photocatalysis. We intend to have a dedicated seminar program for people who are very much involved or are interested in laser spectroscopy.
What can these new lasers do that we were unable to do before?
This is a new system which has a high repetition rate, ten times faster than a standard laser system, allowing for faster experiments with an improved signal-to-noise ratio. The lasers are also very powerful, allowing us to split the output to different detection ranges, so we would be able to investigate the same sample with electronic and vibrational spectroscopy at different timescales under the same laser excitation and within the same facility. It also has the fluorescence upconversion method, which allows you to follow really fast reactions of emissive species with high sensitivity. Very few people have access to this analysis method!
What brought you into photochemistry?
When I was a second year student at Moscow State University, I was involved in evening classes for school children. An academic staff member who coordinated the evening classes asked me what topics I liked, and I said physical chemistry. He asked me to do my research project in his lab, which was on fluorescence. It’s a beautiful area of science and it just felt right. So I came to the laser area through studies of luminescent compounds, which you shine light on and light shines back at you. These can be used in biological imaging and emission sensing, such as determining pressure distribution on aeroplanes.
What brought you to Sheffield?
The University brought me to Sheffield. I did my PhD at Moscow State University and then came to England in 2000 for a year as a Royal Society NATO Fellow. I was then introduced to time-resolved vibrational spectroscopy in Nottingham. I realised I’d like to be in this area so started applying for various jobs and fellowships. Sheffield seemed a very nice place, they had a post advertised and I was offered the job!
Research groups in the department are already using the facility, with multiple research papers published based on the data obtained. Its users range from masters students to
research staff, who perform experiments described below by one of Julia's PhD students, James Shipp.
Transient absorption (TA) is the study of ultrafast processes in the UVvis spectral region. It allows us to easily follow electronic transitions such as metal-to-ligand charge transfer. TA is used in our department for the study of chromophores, such as porphyrins in Dr Adrien Chauvet's group, and for the study of donor-bridge acceptor complexes in the Weinstein group. Research groups in the physics department use TA for the study of advanced photophysical processes, such as triplet-triplet annihilation.
Laser spectroscopies, such as transient absorption, work by a simple pumpprobe method. First, the sample is irradiated with a pump laser pulse. This causes the molecules in the sample to be
promoted to their excited states. To study these excited states we then shine a probe laser on the sample. This allows us to record the absorption spectra of the excited molecules. By increasing the path length of the probe, we can change the difference in time between the two pulses. This allows us to see different time delays, from a few femtoseconds to hundreds of nanoseconds!
Time Resolved Infra-Red
Time resolved infra-red spectroscopy (TRIR) allows us to follow changes in the vibrational spectra of excited molecules over time. This is incredibly useful as many compounds contain functional
groups that strongly absorb IR, such as carbonyl groups. This technique has been used in our department for the study of electron transfer across alkyne bridges in the Weinstein group, as well as the study of mechanisms of photochemical reactions in Dr Peter Portius' group. We can also do sophisticated IR experiments with more than two laser pulses, such as transient two-dimensional IR spectroscopy.
Unlike the other experiments in the laser lab, fluorescence upconversion spectroscopy (FLUPS) does not study a compound’s absorption of light. Instead we collect the light emitted from the excited state, and observe how this emission changes in intensity and wavelength over time. FLUPS is used in the Weinstein group to study emissive platinum complexes, to answer fundamental questions about their electron transfer processes.