Formation of heterogeneous clusters in superfluid helium nanodroplets: phthalocyanine and water.

Clusters consisting of a single phthalocyanine molecule and a single water molecule are investigated by means of electronic spectroscopy in superfluid helium droplets. A recent spectroscopic study of those clusters [J. Fischer, F. Schlaghaufer, E.-M. Lottner, A. Slenczka, L. Christiansen, H. Stapelfeldt, M. Karra, B. Friedrich, T. Mullan, M. Schütz and D. Usvyat, J. Phys. Chem. A, 2019, 123, 10057-10064] which all exhibit a water induced electronic shift to the red is now complemented by the corresponding clusters exhibiting a water induced shift to the blue. These clusters will be analyzed by means of fluorescence excitation spectra, dispersed emission spectra, and additional experimental observations made feasible by helium droplets as cryogenic reactor. Together with the blue shifted clusters a total number of at least 6 isomeric variants could be identified in helium droplets. This contrasts to a number of only three isomeric variants obtained from quantum chemical calculations [J. Fischer, F. Schlaghaufer, E.-M. Lottner, A. Slenczka, L. Christiansen, H. Stapelfeldt, M. Karra, B. Friedrich, T. Mullan, M. Schütz and D. Usvyat, J. Phys. Chem. A, 2019, 123, 10057-10064] disregarding the helium environment and to a single isomer identified in a molecular beam experiment [J. Menapace and E. Bernstein, J. Chem. Phys., 1987, 87, 6877-6889]. The discrepancy in the number of isomers provides evidence of a profound involvement of helium in clustering. Moreover, the discrepancies between the gas phase experiment and quantum chemical calculations similarly reveal the influence of the dynamics of cluster formation on the population of global and local minima that are accessible as isomeric variants.

Electronic spectroscopy of phthalocyanine in a supersonic jet revisited.

Spectroscopic investigation of phthalocyanine in the gas phase has tremendously profited from molecular beam spectroscopy. Isentropic expansion succeeds in reducing the population of rovibrational states to the vibrational ground state so that only low energy rotational states remain populated. However, with respect to UV-vis spectroscopy the pioneers of molecular beam spectroscopy came to the discouraging conclusion that the information contained in the rotational structure of a large molecule is minimal, and even if the rotational structure could be resolved with great effort, the results are unlikely to be worth the difficulty [Levy, Annu. Rev. Phys. Chem., 1980, 31 197-225]. Just over 4 decades later we would like to announce that the result is worth the effort, indeed. Even without full line resolution, the rotational structure at the electronic band origin of phthalocyanine provides deep insight into configurational details of phthalocyanine for both electronic states. These details serve as gas phase compliment to the investigation of microsolvation in superfluid helium droplets. To the best of our knowledge this is the largest molecule ever analyzed by means of its rotational degrees of freedom.

Electronic Spectroscopy of Phthalocyanine and Porphyrin Derivatives in Superfluid Helium Nanodroplets.

Phthalocyanine and porphyrin were among the first organic compounds investigated by means of electronic spectroscopy in superfluid helium nanodroplets. Superfluid helium nanodroplets serve as a very gentle host system for preparing cold and isolated molecules. The uniqueness of helium nanodroplets is with respect to the superfluid phase which warrants the vanishing viscosity and, thus, minimal perturbation of the dopant species at a temperature as low as 0.37 K. These are ideal conditions for the study of molecular spectra in order to analyze structures as well as dynamic processes. Besides the investigation of the dopant species itself, molecular spectroscopy in helium droplets provides information on the helium droplet and in particular on microsolvation. This article, as part of a special issue on phthalocyanines and porphyrins, reviews electronic spectroscopy of phthalocyanine and porphyrin compounds in superfluid helium nanodroplets. In addition to the wide variety of medical as well as technical and synthetical aspects, this article discusses electronic spectroscopy of phthalocyanines and porphyrins in helium droplets in order to learn about both the dopant and the helium environment.

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy.

The empirical model explaining microsolvation of molecules in superfluid helium droplets proposes a non-superfluid helium solvation layer enclosing the dopant molecule. This model warrants an empirical explanation of any helium induced substructure resolved for electronic transitions of molecules in helium droplets. Despite a wealth of such experimental data, quantitative modeling of spectra is still in its infancy. The theoretical treatment of such many-particle systems dissolved into a quantum fluid is a challenge. Moreover, the success of theoretical activities relies also on the accuracy and self-critical communication of experimental data. This will be elucidated by a critical resume of our own experimental work done within the last ten years. We come to the conclusion that spectroscopic data and among others in particular the spectral resolution depend strongly on experimental conditions. Moreover, despite the fact that none of the helium induced fine structure speaks against the empirical model for solvation in helium droplets, in many cases an unequivocal assignment of the spectroscopic details is not possible. This ambiguity needs to be considered and a careful and critical communication of experimental results is essential in order to promote success in quantitatively understanding microsolvation in superfluid helium nanodroplets.

Impulsive laser induced alignment of molecules dissolved in helium nanodroplets.

We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide (CH(3)I) molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets.

Vibrations and hydrogen bonding in porphycene.

Combined use of IR, Raman, neutron scattering and fluorescence measurements for porphycene isolated in helium nanodroplets, supersonic jet and cryogenic matrices, as well as for solid and liquid solutions, resulted in the assignments of almost all of 108 fundamental vibrations. The puzzling feature of porphycene is the apparent lack of the N-H stretching band in the IR spectrum, predicted to be the strongest of all bands by standard harmonic calculations. Theoretical modeling of the IR spectra, based on ab initio molecular dynamics simulations, reveals that the N-H stretching mode should appear as an extremely broad band in the 2250-3000 cm(-1) region. Coupling of the N-H stretching vibration to other modes is discussed in the context of multidimensional character of intramolecular double hydrogen transfer in porphycene. The analysis can be generalized to other strongly hydrogen-bonded systems.

Photolysis of tert-butylthionitrite via excitation to the S(1) and S(2) states studied by 3d-REMPI spectroscopy.

S-Nitrosothiols serve as carriers and donors of NO in several important biological signaling systems. In these compounds the S-NO bond is rather labile and NO can be released thermally or photochemically. This paper reports on the photolytical decomposition of tert-butylthionitrite (t-BuSNO) in the visible and near-UV spectral regions. Between 500 and 605 nm several vibronic levels of the S(1) (npi*) state were excited, including the electronic origin. At 360 nm t-BuSNO is excited near the maximum of the first UV band assigned to the S(2) (pipi*) state. The velocity distributions of several hundred rovibrational states of the NO fragments were recorded with the recently developed 3d-REMPI method. A global fit to these data yields populations of the rovibrational states in both spin-orbit components of the (2)Pi electronic state of NO as well as their velocity distributions and angular anisotropies beta. These data also carry the distribution functions for internal and kinetic energy of the counterfragment, the t-BuS radical. The range found for the anisotropy parameter confirms the npi* character of the visible absorption band (-1.0 < beta < -0.8), and the pipi* character of the UV band (beta = 1.2). Mode-specific dissociation has been observed for excitation into several vibronic bands of the S(0) → S(1) transition. Some produce NO exclusively in the ν = 0 vibrational ground state, whereas some others produce NO almost entirely in the ν = 1 vibrationally excited state. It is concluded that photodissociation is faster than relaxation of the NO stretch vibration of t-BuSNO in S(1) and that it proceeds on purely repulse potential energy surfaces in both electronic states.

Velocity resolved REMPI spectroscopy: a new approach to the study of photodissociation dynamics.

A new data acquisition mode has been implemented to a velocity-map ion-imaging setup, which records the velocity distributions of molecular photofragments with vibrational and rotational resolution. Compared to conventional velocity-map ion-imaging, the acquired data are of remarkable brilliance. This allows for unambiguous assignment of the fragment quantum states and the analysis of all rotational bands apparent in the electronic transition of the molecular fragment. The acquisition time is the same as required for recording a REMPI spectrum of the photofragments. The method is illustrated by the measurement of the rotational state distribution of NO created in the photolytical decomposition of NO(2) at 225 nm. Different rotational distributions were observed for each vibrational state and for each of the four energetically accessible electronic channels.

Rapidly pulsed helium droplet source.

A pulsed valve connected to a closed-cycle cryostat was optimized for producing helium droplets. The pulsed droplet beam appeared with a bimodal size distribution. The leading part of the pulse consists of droplets suitable for doping with molecules. The average size of this part can be varied between 10(4) and 10(6) helium atoms, and the width of the distribution is smaller as compared to a continuous-flow droplet source. The system has been tested in a single pulse mode and at repetition rates of up to 500 Hz with almost constant intensity. The droplet density was found to be increased by more than an order of magnitude as compared to a continuous-flow droplet source.

Mode-selective promotion and isotope effects of concerted double-hydrogen tunneling in porphycene embedded in superfluid helium nanodroplets.

Intramolecular double-hydrogen tunneling in porphycene (see picture) is investigated. Low-temperature conditions are ensured by doping of single molecules into superfluid helium nanodroplets. The investigation of fluorescence excitation and dispersed emission spectra and the highly dissipative environment allows the observation of mode-selective tunneling splitting and reveals a purely concerted tunneling mechanism for all isotopic variants of porphycene.

Structure and dynamics of phthalocyanine-argonn (n = 1-4) complexes studied in helium nanodroplets.

Van der Waals clusters of phthalocyanine with 1-4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett. 2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of 0(0)0 transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine-argon1 complex in helium droplets was estimated to be at most 113 cm-1.

Microsolvation of phthalocyanines in superfluid helium droplets.

Experimental and theoretical investigations of the spectroscopy of molecules in superfluid helium droplets provide evidence for the key role of the first helium layer surrounding the dopant molecule in determining the molecule's spectroscopic features. Recent investigations of emission spectra of phthalocyanine in helium droplets revealed a doubling of all transitions. Herein, we present the emission spectra of Mg-phthalocyanine and of phthalocyanine-argon clusters in helium droplets, which confirm the splitting as a general effect of the helium environment. A scheme of levels is deduced from the emission spectra and attributed to quantized states of the first helium layer surrounding the dopant molecule.