Electronic and vibrational coherences in the B 3 Π(0 + u ) ← X 1 Σ + g transition of chlorine (Cl 2 ) molecules doped in an argon matrix are investigated by exciting with a pulse pair whose relative phase is scanned for every time delay step. The spacing and position of the generated spectral comb are controlled by the delay and relative phase of the pulses. The B 3 Π(0 + u ) ← X 1 Σ + g excitation spectrum shows a progression of zero phonon lines which are decoupled from the lattice and of phonon side bands which originate from a concerted excitation of intramolecular and lattice vibrations. The molecular anharmonicity is compensated for by applying a negative chirp difference of the two pulses, which leads to an asymmetry in the signal around time zero. Absolute phase values are determined from spectral positions in the measured comb patterns. They are used to calculate fringe patterns and to simulate interferograms by a multiplication with the excitation spectrum which are compared with the measured ones in detail. From simulated interferograms of zero phonon lines and phonon side bands, we show that the two contributions can be selected with a large contrast by a time delay of two molecular vibrational periods.
Electronic and vibrational coherences in the B 3 Π(0 + u ) ← X 1 Σ + g transition of chlorine (Cl 2 ) molecules doped in an argon matrix are investigated by exciting with a pulse pair whose relative phase is scanned for every time delay step. The spacing and position of the generated spectral comb are controlled by the delay and relative phase of the pulses. The B 3 Π(0 + u ) ← X 1 Σ + g excitation spectrum shows a progression of zero phonon lines which are decoupled from the lattice and of phonon side bands which originate from a concerted excitation of intramolecular and lattice vibrations. The molecular anharmonicity is compensated for by applying a negative chirp difference of the two pulses, which leads to an asymmetry in the signal around time zero. Absolute phase values are determined from spectral positions in the measured comb patterns. They are used to calculate fringe patterns and to simulate interferograms by a multiplication with the excitation spectrum which are compared with the measured ones in detail. From simulated interferograms of zero phonon lines and phonon side bands, we show that the two contributions can be selected with a large contrast by a time delay of two molecular vibrational periods.
Electronic and vibrational coherences of Cl2 embedded in solid Ar are investigated by exciting to the B state with a phase-locked pulse pair from an unbalanced Michelson interferometer, where the chirp difference matches the B state anharmonicity. Recording the A' --> X fluorescence after relaxation is compared to probing to charge transfer states by a third pulse. The three-pulse experiment delivers more details on the decoherence processes. The signal modulation due to phase tuning up to the third vibrational round-trip time indicates that the electronic coherence in the B <-- X transition is preserved for more than 660 fs in the solid Ar environment where many body electronic interactions take place. Vibrational coherence lasts longer than 3 ps according to the observed half revival of the wavepacket. Control of the coupling between wavepacket motion and lattice oscillation is demonstrated by tuning the relative phase between the phase-locked pulses, preparing wavepackets predominantly composed of either zero-phonon lines or phonon side bands.
With the model system of a diatomic molecule embedded in a rare gas matrix (Br2/Ar), a method is introduced to enhance buried coherent structures and thus to prepare them out of an overwhelming, but incoherent background. Therefore, phase controlled pulse sequences are superimposed, which are generated with a pulse shaper in a direct way based on spectroscopic information. This requires a precise knowledge on the energetic signatures of the target state. This was accomplished via high resolution excitation spectroscopy covering 630 to 540 nm of the X(v′′ = 0)→B(v′) transition. Narrow zero phonon lines (ZPL) were observed with isotopic resolution from v′ = 2 to v′ = 19. They are accompanied by broad phonon sidebands (PSB), which reflect the matrix phonon density of states. With higher excitation energy, a broadening of ZPL and an increase in PSB due to an increase in matrix influence was observed and quantified. Two energetic positions were identified, where repulsive states cross the B-state. The time resolved pump – probe experiments were carried out with ultrashort and thus energetically very broad laser pulses. The B-state dynamics is completely superimposed by A-state dynamics in single pump - probe experiments with 590 nm. However, with the pulse trains the wave packets were created delayed by the B vibrational roundtrip time, which interfere constructively or destructively and thus select spectrally, depending on the relative phase. The required phase adjusted pulse trains were generated by impressing the molecular spectroscopic signature on the spectral composition of the ultrashort pulse. A phase dependent control scheme requires persisting electronic coherence. This was first determined in experiments with double pulses with a Michelson interferometer to at least 1.5 ps, which corresponds to the length of the experimental pulse sequence. In the case of constructive interference due to ZPL excitation, the B vibrational progression builds up and A-state dynamics is suppressed, successfully, since it adds up incoherently to an unmodulated background. Because of this, a surprisingly long coherence of more than 1 ps - also in the PSB - was determined, due to an inverse experiment, based on destructive interference. Since the efficiency of predissociation depends on the coupling strength of the molecule to the matrix, it was specifically attenuated or enhanced by excitation with wave packets which are predominantly composed of ZPL or PSB. Thereby, the dynamical development of predissociation could be determined. My work thus introduces a combination of spectrally sharp defined excitation and time resolved interrogation, which can only be achieved by the coherent superposition of elongated pulse trains. Further on, simulations on the free molecule in first order perturbation theory were carried out, to investigate the temporal behaviour of the interfering wave packets.
Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B(3Π0u) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X(1Σg) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm−1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman packet. This momentum filter, which should be ubiquitous in condensed media, leads to a signal with deeper modulation than in the gas phase.
With a train of 5 phase controlled ultrashort pulses, we enhance a coherent vibronic transition to exceed a dominant incoherent background. The interference in the chromophore generates a spectral comb which is adjusted to a progression of internal Br2 vibrations coupled to phonons of the surrounding Ar lattice. It steers the mode-specific phase evolution of the system. The trains are generated by straightforward programing of the spectral comb in a pulse shaper unit. Excitations involving several hundred degrees of freedom remain coherent on a picosecond time scale.
Calculations are presented demonstrating controlled intermolecular communication between dopant molecules via lattice phonons in a cryogenic matrix. By simulating a time-resolved coherent anti-Stokes Raman scattering (tr-CARS) experiment on an I2-molecule ‘receiver,’ its reception from a Ca-atom ‘transmitter’ of coherent lattice waves in a compressed linear chain of Ar atoms is monitored. A short pulse resonant with an atomic transition in Ca arrives at the sample before the tr-CARS pulses, and the coherent response of the host medium to the electronic excitation sends a rarefaction wave through the lattice. The arrival of this propagating distortion at the I2 molecule is detected through the tr-CARS difference signal, the difference between tr-CARS signals with and without the Ca pulse. A normal-mode analysis of the equilibrium configuration is used to construct an initial Gaussian wave packet, which is propagated by locally quadratic approximations to the relevant many-body nuclear Hamiltonians. Results are presented for both resonant and sub-resonant excitation of the Ca atom. Calculations show a difference signal that turns on abruptly as the lattice wave generated by resonant Ca excitation reaches the I2 chromophore.
Our study of B←X absorption of molecular iodine (I2) isolated in a low-temperature crystalline xenon has revealed an exceptionally long-lived electronic coherence in condensed phase conditions. The visible absorption spectrum shows prominent vibronic structure in the form of zero-phonon lines (ZPLs) and phonon side bands (PSBs). The resolved spectrum implies weak interaction of the chromophore to the lattice degrees of freedom. The coherence extends past the vibrational period of the excited state molecule, unlike that observed in any condensed phase environment for I2 so far. The ZP transitions from the relaxing B-state populations were resolved in the hot luminescence when the 532 nm laser was used for excitation.