Femtosecond Pump-probe Ionization Research Articles

Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents.

Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)1-6, Cy·(MeOH)1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H2O) n are 0.5 ps for n = 1 and ∼0.2-0.25 ps for n = 2-6. For Cy·(MeOH) n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy·THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Gas-Phase Cytosine and Cytosine-N1-Derivatives Have 0.1-1 ns Lifetimes Near the S1 State Minimum.

Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S1 ⇝ S0 internal conversion of its nucleobases. Using picosecond pump-ionization delay measurements, we find that the S1((1)ππ*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼ 700 times longer than that measured by femtosecond pump-probe ionization at higher vibrational excess energy, Eexc. N1-Alkylation increases the S1 lifetime up to τ = 1030 ps for N1-ethyl-Cyt but decreases it to 100 ps for N1-isopropyl-Cyt. Increasing the vibrational energy to Eexc = 300-550 cm(-1) decreases the lifetimes to 20-30 ps. The nonradiative dynamics of S1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N1-substituent. Correlated excited-state calculations predict that the gap between the S2((1)nOπ*) and S1((1)ππ*) states decreases along the series of N1-derivatives, thereby influencing the S1 state lifetime.

Thymine relaxation after UV irradiation: the role of tautomerization and πσ* states

Ab initio calculations and femtosecond pump-probe ionization experiments were carried out to identify excited state relaxation processes in isolated thymine monomer and small thymine-water clusters. Three transient species with life times of < or =100 fs, 7 ps and >1 ns were observed in the experiments on gas phase thymine. The longer-lived transients were weak or absent in thymine-water clusters. Available theoretical results on thymine agree with the assignment of low-lying pi-pi* and n-pi* excited states to the femtosecond and picosecond transients but the assignment of the third transient remains opaque. Our theoretical results seem to exclude the possibility of ground or excited state tautomerization as well as the involvement of states with pi-sigma* character. Remaining explanations for the observed transients are: very fast intersystem crossing to the triplet manifold or the observation of transient signals from local minima on the potential energy surfaces.

Isotope effects in the ultrafast photodissociation of acetone 3s Rydberg state excited at 195 nm

The deuterium isotope effect in the photodissociation of acetone S 2 state was studied using femtosecond pump–probe ionization spectroscopy. The transients obtained for both isotopomers can be well described by the same mechanism in which the primary dissociation occurs on the S 1 surface. Substantial isotope effects were observed in every stage of the reaction. Our results indicted that upon full deuteration the initial-state decay from S 2 to S 1 slows down by a factor of three, the subsequent adiabatic dissociation on the S 1 surface slows down by a factor of four, and the secondary dissociation slows down by a factor of ∼1.6.

Dynamics of Photoinduced Processes in Adenine and Thymine Base Pairs

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.

Studies of electron transfer in NaI with pump–probe femtosecond photoelectron spectroscopy

We discuss an extension of our formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets to nonadiabatically coupled states and present results of its applications to wave packet motion on the ionic (Na+I−) and covalent (NaI) states of sodium iodide. The results of these studies suggest that the energy and angular distributions of these photoelectron spectra provide a useful mapping of the bifurcation of the wave packets through the crossing region and a valuable window on the intramolecular electron transfer occurring between the covalent and ionic states (NaI→Na+I−).

Femtosecond photoionization of RbI

The photodissociation of RbI at 267 nm was studied by femtosecond pump–probe ionization. The probe process of multiphoton ionization by 800 nm resulted in complicated electron and ion signals for dissociation on a purely repulsive curve. The Rb ion signal, produced by dissociative ionization of RbI, has a fast decay and a delayed rise. Transient zero-kinetic-energy electrons were found in the time-resolved photoelectron spectrum, as well as electrons from normal three-photon ionization. Both electron and ion signals were successfully simulated using a classical trajectory method. It is understood that resonance between excited states and their crossings play important roles during the multiphoton ionization probe process.

Femtosecond pump–probe ionization of small NaI–Sn clusters, S:H2O,NH3: A tool to probe the structure of the cluster

Femtosecond pump–probe ionization experiments on small NaI–Sn=1–7 clusters, S:NH3,H2O, are reported. The excitation of the clusters by the pump laser induces the NaI bond breaking within the cluster as well as evaporation of solvent molecules both in the excited and ionic states. Depending on the detection scheme used, namely one-photon or resonant two-photon ionization, the resulting Na+–Sn time-dependent signals are not similar. This reflects the major role of the ionization process in the nature of the dynamics probed. Furthermore, a solvent-selective behavior evidenced on the Na+–Sn signals has been related to different cluster structures according to the solvent nature: NaI seems to be embedded within ammonia clusters while it sticks on the surface of water clusters, when they are larger than four molecules.

Femtosecond energy- and angle-resolved photoelectron spectroscopy

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the Σu+1 double-minimum state of aligned Na2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

Femtosecond energy- and angle-resolved photoelectron spectra

We present energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wavepackets in the 1 Σ u + double-minimum state for aligned Na 2. These results illustrate that a robust description of the underlying photoionization amplitudes can significantly enhance the utility of photoelectron spectroscopy as a probe of wavepacket motion and of the evolution of electronic structure, particularly in cases of avoided crossings and motion over large distances. The angular dependence of these photoelectron spectra provide insightful fingerprints of vibrational wavepacket dynamics and can be a useful real-time probe of molecular rotation.

Quantum dynamical three-dimensional ab initio approach to a femtosecond pump-probe ionization spectrum of Na 3(B) at low laser field intensities

The transient femtosecond pump-probe ionization spectrum of Na 3 in its B state is simulated for low laser field intensities. All three vibrational degrees of freedom of Na 3 have been taken into account in order to calculate a pump-probe spectrum using ab initio potential energy surfaces. The time-dependent wavepacket dynamics elucidate the excitation mechanism during the pump-probe experiment. The dominant oscillations in the ion signal correspond to the motion of a coherent wavepacket along the Q s mode. The theoretical and experimental pump-probe ionization spectra are found to be in good agreement.

Model studies on femtosecond time-resolved ionization spectroscopy of excited-state vibrational dynamics and vibronic coupling

The time-resolved detection of excited-state vibrational motion and vibronic coupling with femtosecond pump-probe ionization spectroscopy is analysed within the framework of lowest-order perturbation theory with respect to the radiation-matter interaction. It is shown for a simple model of shifted harmonic oscillators in the Condon approximation that coherent wave packet motion in the excited electronic state is reflected in the ion-yield signal when laser pulses of suitable duration are employed (neither too short nor too long). It is furthermore shown that the pronounced oscillatory electronic population dynamics of adiabatic electronic states induced by vibronic coupling can be monitored almost perfectly by pump-probe ionization with suitable pulses. In both cases we analyze the conditions (pulse durations and laser frequencies) under which the phenomena manifest themselves in the time-resolved ion-yield signal.