Tracking ultrafast dynamics by sub-20-fs UV pulses generated in the lab open atmosphere

This study describes a simple method to generate sub-20 fs UV-pulses (264 nm) by third-harmonic generation, in an air-plasma filament formed after focusing the fundamental 800 nm beam directly in the lab open-atmosphere. The generated pulses are applied to track the relaxation through the conical intersection that couples the S2 and S1 states, in the benchmark molecule of naphthalene. The transients, with a resolution of about 25 fs, show two differentiate patterns of quantum beats. The assignation of these oscillations to specific modes in the lower S1 state and to electronic coherence between the two coupled states is discussed

METTL1 promotes tumorigenesis through tRNA-derived fragment biogenesis in prostate cancer

Newly growing evidence highlights the essential role that epitranscriptomic marks play in the development of many cancers; however, little is known about the role and implications of altered epitranscriptome deposition in prostate cancer. Here, we show that the transfer RNA N-7-methylguanosine (m(7)G) transferase METTL1 is highly expressed in primary and advanced prostate tumours. Mechanistically, we find that METTL1 depletion causes the loss of m(7)G tRNA methylation and promotes the biogenesis of a novel class of small non-coding RNAs derived from 5'tRNA fragments. 5'tRNA-derived small RNAs steer translation control to favour the synthesis of key regulators of tumour growth suppression, interferon pathway, and immune effectors. Knockdown of Mettl1 in prostate cancer preclinical models increases intratumoural infiltration of pro-inflammatory immune cells and enhances responses to immunotherapy. Collectively, our findings reveal a therapeutically actionable role of METTL1-directed m(7)G tRNA methylation in cancer cell translation control and tumour biology

COHERENT EFFECTS INVOLVING THE EXCITATION AND RELAXATION OF THE COUPLED La_a/Lb_b ELECTRONIC EXCITED STATES OF NAPHTHALENE: A TIME DEPENDENT EXPERIMENTAL STUDY

Author Institution: Departamento de Quimica-Fisica, Facultad de Ciencia; y Tecnologia, Universidad del Pais Vasco, Apartado 644, ES-48080; Bilbao, SpainThe ultrafast dynamics of the non-adiabatic coupling between the L$_a$(S$_2$)/L$_b$(S$_1$) electronic excited states of naphthalene has been investigated, regarding the coherent aspects of the preparation and subsequent relaxation of the system. This electronic coupling represents a well known case of non-adiabatic behavior that has been used for years as a benchmark to test theoretical models. The jet cooled naphthalene molecule was prepared in the L$_a$ and L$_b$ states by femtosecond pump pulses in the UV region (318-268 nm), while the temporal evolution of the system was tracked by multiphoton ionization of the molecule with probe pulses centered at 800 nm. The time dependent signals collected at excitation wavelengths corresponding to the weak S$_0$-L$_b$ transition are dominated by the CPR (Coherent Population Transfer) effect induced by the blue-shifted stronger S$_0$-L$_a$ absorption. The CPR effect results in the transient population of the L$_a$ state during the interaction of the pump pulse with the system, revealing the coherent nature of the excitation process. The transients collected after excitation to the L$_a$ state reveal the ultrafast relaxation to the strongly coupled L$_b$ state. The electronic and/or vibrational nature of the periodic recurrences observed along the relaxation of the electronic population will be discussed

COHERENT EXCITATION PHENOMENA IN TIME-RESOLVED EXPERIMENTS

Author Institution: Departamento de Quimica-Fisica, Facultad de Ciencia; y Tecnologia, Universidad del Pais Vasco, Apartado 644, ES-48080; Bilbao, SpainThe influence of coherent phenomena on femtosecond pump-probe experiments in molecular systems has been investigated. The signature of Coherent Population Return (CPR) has been observed, and satisfactorily described by means of a coherently excited two-state model. This analysis has been extended to a more general situation where N two-level subsystems interact independently with the radiation. The obtained results permit us to explain why for such complex systems the incoherent treatment provides an accurate description of the population dynamics. Furthermore, the distribution of states inside and outside the excitation laser bandwidth, rather than the state density as it is accepted, is found to be the key parameter for determining the applicability of the incoherent approach

Influence of coherent adiabatic excitation on femtosecond transient signals

Femtosecond laser pulses are the tools of choice for inducing and tracking the temporal evolution of electronic excitation in molecular systems. To obtain this information, a proper theoretical modeling of the observables monitored in these experiments is essential. Herein, we present a coherent approach to simulate the time-dependent signals that result from femtosecond pump-probe experiments with ionization detection. Thus, the transient signals derived from femtosecond pump-probe experiments are analyzed in terms of the coherent evolution of the energy levels perturbed by the excitation pulse. The model system is treated as the sum of independent two-level subsystems that evolve adiabatically or are permanently excited, depending on the detuning from the central wavelength of the excitation laser. This approach will allow us to explain numerically and analytically the convergence between the coherent and incoherent (rate equations) treatments for complex multi-level systems. It will be also shown that the parameter that determines the validity of the incoherent treatment is the distribution of states outside and inside the laser bandwidth, rather than the density of states as it is commonly accepted