FemtoMAX - an X-ray beamline for structural dynamics at the short-pulse facility of MAX IV.

The FemtoMAX beamline facilitates studies of the structural dynamics of materials. Such studies are of fundamental importance for key scientific problems related to programming materials using light, enabling new storage media and new manufacturing techniques, obtaining sustainable energy by mimicking photosynthesis, and gleaning insights into chemical and biological functional dynamics. The FemtoMAX beamline utilizes the MAX IV linear accelerator as an electron source. The photon bursts have a pulse length of 100 fs, which is on the timescale of molecular vibrations, and have wavelengths matching interatomic distances (Å). The uniqueness of the beamline has called for special beamline components. This paper presents the beamline design including ultrasensitive X-ray beam-position monitors based on thin Ce:YAG screens, efficient harmonic separators and novel timing tools.

Spatiotemporal characterization of ultrashort laser pulses using spatially resolved Fourier transform spectrometry.

We present a method for characterizing ultrashort laser pulses in space and time, based on spatially resolved Fourier transform spectrometry. An unknown pulse is interfered with a delayed, spatially uniform reference on a CCD camera. The reference pulse is created by spatially filtering a portion of the unknown pulse. By scanning the delay between the two pulses, an interferogram is obtained at each pixel, allowing us to determine the spatially resolved phase difference between the unknown pulse and the reference pulse. High-resolution spatiotemporal characterization of an ultrashort pulse is demonstrated, and the sensitivity of the method to spatiotemporal coupling is shown for the case of a pulse with pulse front tilt.

Ultrafast relaxation dynamics of uracil probed via strong field dissociative ionization.

We study the ultrafast relaxation dynamics of uracil excited to the first bright ππ* state (S2) by an ultrafast laser pulse in the deep ultraviolet (central wavelength λ0 = 260 nm). With a unique combination of strong field dissociative ionization measurements, state of the art strong field ionization calculations, and high level ab initio calculations of excited neutral and ionic states at critical points along the neutral potentials, we are able to gain a detailed picture of the relaxation dynamics of the molecule, which resolves earlier disagreements regarding measurements and calculations of the relaxation.

The influence of excited state topology on wavepacket delocalization in the relaxation of photoexcited polyatomic molecules.

This paper compares the relaxation dynamics of several molecules that display internal conversion on ultrafast time scales. We find that the degree of wavefunction localization during relaxation is strongly correlated with the rate of relaxation. We discuss our experimental findings in terms of two-dimensional model simulations which try to capture the essential features of the potential energy landscapes relevant to the relaxation dynamics. Our model calculations show how relaxation can be local or nonlocal depending on basic features of the potential energy surface traversed by the wavepacket en route back to the ground state.

Neutral-ionic state correlations in strong-field molecular ionization.

We study correlations between neutral and ionic states in strong-field molecular ionization. We compare predictions based on Dyson orbital norms and quasistatic semiclassical tunneling theories (Keldysh and molecular orbital Ammosov-Delone-Krainov) with more detailed calculations of strong-field ionization which take into account (i) the Coulomb interaction between the outgoing continuum electron wave packet and the remaining bound electrons and (ii) electron-core interactions that cause distortions of the electronic continuum states during the ionization event. Our results highlight the prominence of electronic rearrangement effects in strong-field ionization with intense ultrafast laser pulses, where the outgoing continuum electron can cause electronic transitions in the parent ion. Calculations and measurements for excited uracil molecules reveal the breakdown of Keldysh-weighted Dyson norm predictions for ionization to different states of the molecular cation in the strong-field regime.

Fragmentation pathways in the uracil radical cation.

We investigate pathways for fragmentation in the uracil radical cation using ab initio electronic structure calculations. We focus on the main fragments produced in pump-probe dissociative ionization experiments. These are fragments with mass to charge ratios (m/z) of 69, 28, 41, and 42. Barriers to dissociation along the ground ionic surface are reported, which provide an estimate of the energetic requirements for the production of the main fragments. Direct and sequential fragmentation mechanisms have been analyzed, and it is concluded that sequential fragmentation after production of fragment with m/z 69 is the dominant mechanism for the production of the smaller fragments.

Two-dimensional fourier transform spectroscopy of adenine and uracil using shaped ultrafast laser pulses in the deep UV.

We compare two-dimensional (2D) ultrafast Fourier transform spectroscopy measurements in the deep UV (262 nm) for adenine and uracil in solution. Both molecules show excited-state absorption on short time scales and ground-state bleaching extending for over 1 ps. While the 2D spectrum for uracil shows changes in the center of gravity during the first few hundred femtoseconds, the center of gravity of the 2D spectrum for adenine does not show similar changes. We discuss our results in light of ab initio electronic structure calculations.

Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: a case study of cytosine.

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S(0)-S(1) resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.

Combining dissociative ionization pump-probe spectroscopy and ab initio calculations to interpret dynamics and control through conical intersections.

Nonadiabatic processes play an important role in molecular dynamics, and understanding these processes better can help interpret and guide control over molecules. We are using high level electronic structure calculations in combination with intense, shaped, ultrafast laser pulses to study excited state dynamics in the nucleic acid bases, cytosine and uracil. These molecules have very short excited state lifetimes as they relax radiationless through conical intersections after absorption of UV radiation. The presence of more than one relaxation pathway provides the possibility to control which pathway can be involved in the dynamics. In our approach the molecules were excited using ultrafast laser pulses in the deep UV and then probed with strong field near infrared pulses which ionize and dissociate the molecules. Key to this approach is the fact that different fragments exhibit different dynamics and we can correlate these fragments, and their associated dynamics, to the various pathways involved in the neutral dynamics. Multiconfigurational electronic structure methods were used to calculate potential energy surfaces of the neutral and ionic states involved in the dynamics. Calculating mechanisms for fragmentation in the ion enables us to relate specific fragments to different neutral pathways, and use them as signatures to follow the dynamics. Possibilities for control are also discussed.

Closed-loop learning control of isomerization using shaped ultrafast laser pulses in the deep ultraviolet.

We demonstrate the use of shaped ultrafast laser pulses in the deep ultraviolet to control the ring opening isomerization of 1,3-cyclohexadiene to form 1,3,5-hexatriene. The experiments are performed with a gas phase sample and the isomerization yield is probed with dissociative ionization driven by a time-delayed, intense infrared laser pulse. Differences in the electronic structure of the ions for the two isomers, as shown by ab initio calculations, result in very different fragmentation products following strong-field ionization. We find that a shaped pulse yields a approximately 37% increase in the isomerization over an unshaped laser pulse.