Abstract
We determined the complete relaxation dynamics of pyrene in ethanol from the second bright state, employing experimental and theoretical broadband heterodyne detected transient grating and two-dimensional photon echo (2DPE) spectroscopy, using pulses with duration of 6 fs and covering a spectral range spanning from 250 to 300 nm. Multiple lifetimes are assigned to conical intersections through a cascade of electronic states, eventually leading to a rapid population of the lowest long-living excited state and subsequent slow vibrational cooling. The lineshapes in the 2DPE spectra indicate that the efficiency of the population transfer depends on the kinetic energy deposited into modes required to reach a sloped conical intersection, which mediates the decay to the lowest electronic state. The presented experimental–theoretical protocol paves the way for studies on deep-ultraviolet-absorbing biochromophores ubiquitous in genomic and proteic systems.
Highlights
Insight into photoactivated biological functions requires indepth knowledge of rapid molecular processes
The lineshapes in the 2DPE spectra indicate that the efficiency of the population transfer depends on the kinetic energy deposited into modes required to reach a sloped conical intersection, which mediates the decay to the lowest electronic state
This can be achieved through dynamically resolved spectroscopies that recently became feasible in the deep ultraviolet (UV) below 300 nm.[1−5] Because of enormous challenges in generating broadband laser pulses, two-dimensional photon echo (2DPE) and conventional transient absorption (TA) and transient grating (TG) experiments in the deep-UV are conducted in a one-color fashion with small bandwidth excitation pulses, limiting temporal resolution and informative value, and the informative value of ultrafast processes on the subpicosecond time scale is limited by the temporal resolution
Summary
Labeled 2B3u (second state in the B3u irreducible representation of D2h symmetry). A peaks at 272 nm (36.7 well-resolved kcm−1), 262 vibronic nm (38.2 progression with kcm−1), and 252 nm (39.7 kcm−1) is observed.[26]. Resonance Raman spectroscopy of the 1B2u,26 as well as previously reported vibrationally resolved LA simulations,[28] reveal that the same high-frequency (1456 cm−1) and lowfrequency (405 cm−1) modes are responsible for the 1B2u band vibrational progression and broadening. We observe an intense positive band at 272 nm associated with the bleach of the fundamental, together with two negative ESA signatures at 255 and 265 nm. The 0.75 ps lifetime is associated with the initial increase of the ESA band intensity, and it is accompanied by the decrease of the positive 272 nm signal, followed by vibrational cooling with an 8 ps lifetime which blue-shifts the maxima of the contributions. The extracted frequencies match those from the normal-mode analysis of the 2B3u state.[26,27] Because of the overlap of bleach and photoinduced absorption in the probed window, the assignment of the Raman modes is not unambiguous and we do not exclude the possibility of
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