Modelling laser induced temperature-jumps in ZSM-5 and developing a new T-jump laser Amy Edmeades, Alexander Hawkins, Mike Towrie and Paul M. Donaldson Central Laser Facility, UK Time resolved temperature-jump (T-jump) IR spectroscopy using pulsed laser techniques can be used to study thermally activated processes such as DNA melting and protein unfolding.[1,2] Nanosecond heating can be achieved, followed by cooling times on the microsecond to millisecond time scale.[3,4] It is possible to use intensity modulated CW laser light to sustain the temperature of the sample for seconds after initial heating [5]. This slows the heating time to <;1ms, but extends the range of timescales that are unaffected by cooling of the sample. Our interest in exploring these techniques is for studying chemistry in catalytic systems. At the Ultra facility at CLF, we have delivered 70µJ pulses to zeolite (ZSM-5) pellets generating 50-100 o C T-jumps with a single 1ns laser shot, with subsequent cooling observed on a millisecond timescale.[6] Using multiple laser shots at 1kHz, we have achieved higher, more sustained T-jumps through a stepped heating process that spans many milliseconds. In this poster, we present physical modelling of the heating and cooling process, and a new T-Jump laser based on a nanosecond pulsed fiber pumping a signal-resonant PPLN OPO. The fiber laser is capable of producing 4-2000ns pulses at 1-4000kHz. Parametrically downconverted idler pulse energies up to 100uJ have been observed from 2-5µm. This new laser is capable of inducing greater heating on a microsecond timescale than our original 1ns 1kHz system.[3] Delivering this energy in longer pulses has the benefit of lowering peak intensity, allowing higher T-jumps to be achieved without reaching the threshold for plasma generation. The option to vary the pulse duration and repetition rate provides the opportunity to design pulse sequences as needed for desired heating and cooling times. For characterising the T-jump in the zeolite samples, the temperature distributions and their evolution over time are better understood through simulations.[1,2] Modelling based on the heat diffusion equation and realistic experimental parameters is presented here. Inhomogeneous spatial temperature profiles are observed due to absorption of the laser beam through the thickness of the sample. The results of these simulations provide predictions for attainable T-jumps and associated cooling times, for single pulses as well as bursts of pulses, supporting understanding of collected T-jump data as well as design of future experiments. References 1. R. Fritzsch, G. Greetham, I. Clark, L. Minnes, M. Towrie, A. Parker, N. Hunt, Monitoring Base-Specific Dynamics during Melting of DNA-Ligand Complexes Using Temperature-Jump Time-Resolved Infrared Spectroscopy, J. Phys. Chem. B 123 (29), 6188–6199 (2019) 2. C. Phillips, Y. Mizutani, R. Hochstrasser, Ultrafast thermally induced unfolding of RNase A. PNAS 92(16), 7292-7296 (1995) 3. G. Greetham, I. Clark, B. Young, R. Fritsch, L. Minnes, N. Hunt, M. Towrie, Time-Resolved Temperature-Jump Infrared Spectroscopy at a High Repetition Rate, Applied Spectroscopy 74(6), 720-727 (2020) 4. H. Chung, M. Khalil, A. Smith, A. Tokmakoff, Transient two-dimensional IR spectrometer for probing nanosecond temperature-jump kinetics, Rev. Sci. Instrum 78, 063101 (2007) 5. B. Ashwood, N. Lewis, P. Sanstead, A. Tokmakoff, Temperature-Jump 2D IR Spectroscopy with Intensity-Modulated CW Optical Heating, J. Phys. Chem. B 124 (39), 8665-8677 (2020) 6. A. Hawkins, A. Edmeades, C. Hutchison, G. Greetham, M. Towrie, R. Howe, P. Donaldson, Laser induced temperature-jump time resolved IR spectroscopy of zeolites, In preparation (2023)
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