Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol−1. This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding. DOI: 10.1103/PhysRevLett.111.153002 PACS numbers: 33.80.Àb, 36.40.Àc, 67.25.dw Analogous to superconductivity, superfluidity is a macroscopic manifestation of quantum mechanics. It derives its name from the frictionless flow of a liquid [1,2]. While superfluidity has been observed for Bose-Einstein condensates [3] and more recently for polaritons, [4] helium is undoubtedly the best-known example of a superfluid. The peculiar dispersion curve of He dictates that an object moving through superfluid helium can only create elementary excitations if its speed exceeds the so-called critical Landau velocity of $58 m=s [5,6]. Whereas the critical Landau velocity could be experimentally verified in bulk helium, [7] its manifestation in finite size helium systems is still matter of debate [8][9][10]. Knowledge of such a fundamental property becomes essential as finite size helium systems, in the form of helium nanodroplets, are increasingly being used as a matrix for a wide variety of studies [11][12][13].Many properties of helium nanodroplets have been characterized during the last two decades using solvated molecules as spectroscopic probes [14]. Vibrational spectroscopy of solvated carbonyl sulfide (OCS) has provided evidence for microscopic superfluidity in these finite size systems [15]. While a clearly resolved rotational structure was observed in the IR absorption spectrum of OCS in 4 He droplets, this structure was markedly absent in 3 He droplets, which are not superfluid due to their fermionic character. In contrast, the temporal evolution of rotational wave packets of methyliodide molecules dissolved in helium droplets has recently been found to differ dramatically from that of isolated molecules [16]. This raises the question to what extent microscopic superfluidity can be related to the frictionless flow of superfluid helium. Here, we present an approach that uses the translational motion of electronically excited atoms and molecules to probe superfluidity in helium nanodroplets and to establish the existence of a critical velo...

Infrared spectroscopy provides a means to determine the intrinsic geometrical structures of molecules. Here we present a novel spectroscopic method that uses superfluid helium nanodroplets to record IR spectra of cold molecular ions, in this particular case aniline cations. The method is based on the detection of ions that are ejected from the helium droplets following vibrational excitation of these ions. We find that spectra can be recorded with a high sensitivity and that they exhibit only a small matrix shift. The widths of the individual transitions depend on the excited vibrational level and are thought to be related to the interaction of the ion with the surrounding helium solvent shells.

We report on the first successful high-resolution spectroscopic studies on isolated para-coumaric acid, the chromophore of the photoactive yellow protein which has become a model system for studying biological light-induced signal transduction. Employing various double-resonance multiphoton ionization techniques in combination with mass-resolved ion detection and the results of quantum chemical calculations, we identify three conformations the molecule can adopt under our experimental conditions. The vibrational activity in the excitation spectra allows us to conclude that in the Franck-Condon region accessed from the ground state S(1) is the V'(pipi*) state. Interestingly, we find considerable out-of-plane vibrational activity, indicating that the molecule adopts a nonplanar geometry in S(1). The ionization requirements show that after excitation rapid internal conversion takes place to a lower-lying npi* state. Such a state has been postulated by ab initio calculations on para-coumaric acid and derivatives, but until the present study no direct evidence had been found for its presence.

High-resolution Resonance Enhanced MultiPhoton Ionization (REMPI) and Laser Induced Fluorescence (LIF) excitation spectra of jet-cooled methyl-4-hydroxycinnamate, methyl-4-OD-cinnamate, and of their water clusters have been recorded. Whereas water complexation leads to significant linewidth narrowing, isotopic substitution does for all practical purposes not influence the excited-state dynamics. In this light, we evaluate two previously proposed decay channels of the photoexcited ππ* state involving the dissociative πσ* state (analogous to phenol) and involving the optically dark nπ* state (as concluded for para-coumaric acid). To come to an unambiguous interpretation of the REMPI studies, it has been necessary to determine ionization thresholds. For methyl-4-hydroxycinnamate and its water cluster values of 8.078 and 7.636 eV have been found. Apart from the electronic excitation studies, IR absorption studies have been performed as well. These studies provide important vibrational markers for the assignment of the various conformations that are present under molecular beam conditions, and offer a direct measure of the influence of hydrogen bonding on the properties of the hydroxyl group.

Excitation spectra of the Ã2A2←X̃2B1 and B̃2B1←X̃2B1 transitions of aniline cations embedded in helium nanodroplets are reported. The spectra are characterized by broad asymmetric resonances that consist of an intrinsically broadened zero-phonon line, which partially overlaps with the accompanying phonon wing. The band origin of the Ã2A2←X̃2B1 transition reveals a blue-shift of 116 ± 84 cm–1 with respect to the corresponding gas phase transition. This blue-shift is suggested to be related to the presence of a relatively strongly bound high-density helium solvation layer around the ion.

UV excitation and IR absorption spectroscopy on jet-cooled molecules is used to study the conformational heterogeneity of methyl 4-hydroxycinnamate, a model chromophore of the Photoactive Yellow Protein (PYP), and to determine the spectroscopic properties of the various conformers. UV-UV depletion spectroscopy identifies four different species with distinct electronic excitation spectra. Quantum chemical calculations argue that these species are associated with different conformers involving the s-cis/s-trans configuration of the ester with respect to the propenyl C-C single bond and the syn/anti orientation of the phenolic OH group. IR-UV hole-burning spectroscopy is used to record their IR absorption spectra in the fingerprint region. Comparison with IR absorption spectra predicted by quantum chemical calculations provides vibrational markers for each of the conformers, on the basis of which each of the species observed with UV-UV depletion spectroscopy is assigned. Although both DFT and wave function methods reproduce experimental frequencies, we find that calculations at the MP2 level are necessary to obtain agreement with experimentally observed intensities. To elucidate the role of the environment, we compare the IR spectra of the isolated conformers with IR spectra of methyl 4-hydroxycinnamate-water clusters, and with IR spectra of methyl 4-hydroxycinnamate in solution.

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