Electronic transient spectroscopy from the deep UV to the NIR: unambiguous disentanglement of complex processes

Abstract

Complex multi-stage relaxation and reaction pathways after the optical excitation of molecules makes the disentanglement of the underlying mechanisms challenging. We present four examples that a new transient spectrometer with excitation fully tunable from the deep UV to the IR and 225 to 1700 nm probing allows for an analysis with greatly reduced ambiguity. The temporal resolution of about 50 fs allows us to resolve all relevant processes. For each example there is a new twist in the sequence of relaxation steps that had previously been overlooked. In malachite green it appears that the importance of the phenyl twisting has been overemphasized and rather a charge transfer state should be considered. In TINUVIN-P the predicted twisting as the driving motion for the ultrafast IC is confirmed and leads to a resolution of the earlier puzzle that the sub-5 ps regime shows kinetics deviating from a pure cooling process despite the sub-ps proton transfer cycle. For the bond cleavage of Ph 2 CH-Cl and Ph 2 CH-Br the degree of electron transfer within the radical pair can now be determined quantitatively and leads to a profound understanding of the long term cation yield. For the first time coherent wavepacket motion in the photoproducts is reported. Last but not least the measurement of the GSB recovery in the deep UV allows for the surprising result, that even after S 2 excitation of cyclopentenones the triplet states are reached with near unity probability within a few picoseconds.

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

“…Recent advancement toward this goal has led to some improvement in accessing the UV region down to 230 nm. 25 For the present study, we have implemented femtosecond transient absorption with a continuum probe detection capability in the deep UV ranging from 220 to 330 nm (Scheme 2). This enables us to investigate the excited-state dynamics of the above-mentioned symmetrical β-diketones and opens an avenue for other compounds as well.…”

Ultrafast supercontinuum generation in bulk condensed media

“…Consequently, these media exhibit extremely broad transparency windows, which extend from the vacuum ultraviolet to the mid-infrared, making them attractive nonlinear materials for SC generation as pumped with femtosecond laser pulses at various parts of the optical spectrum. In particular, lithium and calcium fluorides, LiF and CaF 2 , are the only nonlinear media that are able to produce an appreciable spectral broadening in the deepultraviolet, as demonstrated using the third harmonic pulses from the Ti:sapphire laser [34,146]. When pumped with the second harmonic pulses from the same laser, the measured SC spectra in these media cover the ultraviolet and visible spectral ranges [146,183,185].…”

“…An extremely broad SC spectrum is on demand for transient pump-probe spectroscopy and gives an access to study ultrafast molecular dynamics and processes in the condensed matter with femtosecond time resolution (see, e.g. [27,[32][33][34]). The use of SC for Z-scan measurements enables a broadband characterization of the nonlinear absorption spectra [35,36].…”

Tracking a Paternò–Büchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies

“…[21][22][23][24] The present study has two aims: to gain new and direct insight into this important reaction (extracting quantum yields of intermediate and product formation wherever possible) and to illustrate the role of contemporary ultrafast transient absorption spectroscopy methods in unravelling mechanistic details of a classic photoinduced organic reaction.…”