Uzi Even

Cooling of large molecules below 1 K and He clusters formation

We present here the design details of a high-pressure pulsed valve that generates intense supersonic jets. The measured rotational contours of Aniline indicate that temperatures lower than 0.5 K can be achieved before the formation of clusters with the He carrier gas. The spectral shifts and vibronic structure of Anthracene–Hen clusters (n=1–6) are showing some surprising features.

Microscopic solvation effects on excited‐state energetics and dynamics of aromatic molecules in large van der Waals complexes

In this paper we report the results of an experimental study of the formation kinetics, excited‐state energetics, interstate electronic relaxation, and intrastate nuclear dynamics in electronically–vibrationally excited states of van der Waals molecules, consisting of a tetracene (T) molecule and rare‐gas (R) atoms. The TRn molecules were synthesized in seeded supersonic jets. Excited‐state energetics and dynamics of TRn molecules were explored by laser spectroscopy in supersonic expansions, interrogating the fluorescence action spectra, the energy‐resolved emission, the relative emission quantum yields, and the time‐resolved emission. Spectroscopic diagnostic methods for the identification and characterization of the chemical composition of TRn complexes involved the dependence of the spectral features on the identity of the rare gas, an intensity conservation rule for the intensities of TArn and of T, the pressure dependence of the intensity of the bare T molecule, the pressure dependence of the intensi...

Cooling of large and heavy molecules in seeded supersonic beams

In this paper we report the application of the techniques of laser fluorescence excitation spectroscopy for the experimental study of the rotational—vibrational cooling of iodine (I2) and of several large molecules, i.e. anthracene (C14H10), tetracene (C18H12), pentacene (C22H14) and ovalene (C32H14), in seeded supersonic beams of rare gases. We have found that the mass of the rare gas expanded at stagnation pressures of p = 20–8300 Torr and through a nozzle of diameter D = 50–200 μ exhibits a marked effect on the degree of rotational—vibrational cooling of the large molecules. The degree of internal cooling increases in the order He < Ne < Ar < Kr < Xe. Cooling of large and heavy molecules in a supersonic expansion of heavy diluents down to a rotational temperature Tr ≈ 5–7 K and vibrational temperature TV < 50 K can be accomplished at moderate values of pD ≈ 2.4–3.0 Torr cm for Ar, pD ≈ 2.0 Torr cm for Kr and pD ≈ 1.4 Torr cm for Xe. Effective internal cooling at moderate values of pD cannot be accomplished in light diluents, i.e., He and Ne, in view of the velocity slip effect. The degree of rotational cooling of large molecules in heavy diluents, such as Ar, Kr and Xe, seems to be as efficient as that of I2 in these media. On the other hand, the degree of vibrational cooling of large molecules in Ar, Kr and Xe is very efficient, in marked contrast to the ineffective vibrational cooling of I2 in heavy diluents under the same circumstances. We have observed that the effective formation of van der Waals molecules between the aromatic molecule and Ar, Kr and Xe is exhibited only after vibrational sequence congestion of the large molecule is eliminated. These observations provide an experimental basis for the use of seeded beams of Ar, Kr and Xe at moderate values of pD for the interrogation of excited-state energetics and dynamics of internally cold, isolated, bare, large molecules. Finally, we demonstrated the possibility of performing laser spectroscopy of large molecules in high-flow seeded supersonic beams expanded through a nozzle for D = 150 μ at p = 10 atm, employing a primitive pumping system based on a mechanical pump without using diffusion pumps.

Energetics and intramolecular dynamics of the isolated ultracold tetracene molecule in its first excited singlet state

In this paper we report the results of an experimental study of the energetics and intramolecular dynamics of the first electronically excited S1 singlet state of tetracene (C18H12) seeded in supersonic expansions of rare gases. Internal cooling of tetracene in supersonic expansions of Ar down to rotational temperatures TR∼5–7 K and vibrational temperatures TV<50 K can be accomplished at moderately low stagnation pressures p = 100–200 Torr of Ar, Kr, and Xe when expanded through a D = 150 μ nozzle. We have interrogated the fluorescence action spectra, the energy‐resolved fluorescence, and the time‐resolved fluorescence of the ultracold, isolated, bare, large molecule for excess vibrational energies EV = 0–4000 cm−1 above the electronic origin of the S1 state. The electronic origin of the S0(1A1g)→S1(1B2u) transition located at 22 360 cm−1 exhibits a partially resolved B‐type rotational structure, which concurs with the short axis polarization of this transition. We were able to identify nine totally symme...

Stopping Supersonic Beams with a Series of Pulsed Electromagnetic Coils : An Atomic Coilgun

We report the stopping of an atomic beam, using a series of pulsed electromagnetic coils. We use a supersonic beam of metastable neon created in a gas discharge as a monochromatic source of paramagnetic atoms. A series of coils is fired in a timed sequence to bring the atoms to near rest, where they are detected on a microchannel plate. Applications to fundamental problems in physics and chemistry are discussed.

Fluorescence excitation spectra of indole, 3‐methyl indole, and 3‐indole acetic acid in supersonic jets

In this paper we report on the laser‐induced fluorescence spectra of indole, 3‐methyl indole, and 3‐indole acetic acid in pulsed supersonic expansions of He. The electronic origin of the first excited (1Lb) state is 35 233 cm−1 for indole, 34 887 cm−1 for 3‐methyl indole, and 34 044 cm−1 for 3‐indole acetic acid. On the basis of the energetics and fluorescence lifetimes, the spectral features at excess vibrational energies EV=0–2200 cm−1 for indole and EV=0–1600 cm−1 for 3‐methyl indole were assigned to the vibrational level structure within a single electronic manifold. The second electronically excited (1La) state is not located in the above energy ranges. The energetics of selective solvation of 3‐methyl indole was inferred from the spectra of the van der Waals complexes with Ar, H2O, and D2O, which provide information on microscopic solvent shifts of the electronic origin and of some prominent vibrational excitations of the 1Lb states.

Electronic spectral shifts of aromatic molecule-rare-gas heteroclusters

In this paper, a semiempirical theory for the spectral shifts of the electronic origin of the S0→S1 transition of (aromatic molecule)⋅(rare‐gas)n heteroclusters is advanced and applied. Neglecting the modifications of intermolecular overlap and exchange interactions upon electronic excitation, the dispersive contributions to the spectral shift are evaluated to second order, accounting for finite‐size structural features of the large molecule by the utilization of the multicenter monopole representation of the intermolecular interactions. The spectral shifts for nonpolar aromatic hydrocarbons in or on rare‐gas heteroclusters are represented in terms of differences between electrostatic interactions involving an electrostatic field (due to the molecular transition monopoles charge distribution) and an induced dipole (originating from the rare‐gas polarizability). The transition monopoles incorporated all the one‐ and two‐electron ππ* excitations of the aromatic molecule, which were represented by Huckel or ...

Isolated ultracold porphyrins in supersonic expansions. III. Free‐base porphine

The vibrational level structure of the S0→S1x transition (the Qx band) and of the S0→S1y transition (the Qy band) of free‐base porphine in supersonic expansions of He was interrogated by laser‐induced fluorescence excitation spectroscopy. Electronic relaxation in the S1x manifold was explored by time‐resolved spectroscopy, revealing a constant value of the decay lifetime τ=9.5±1.0 ns for excess vibrational energies in the range Ev=0–5000 cm−1. The line broadening (FWHM) Δ=1.0–1.5 cm−1 of the electronic origin and of low‐lying vibrational excitations in the S1x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ=11.9 cm−1 of the electronic origin of the S1y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S1y was found to be Lorentzian, providing a quantitative determination of the lifetime τ=5×10−13 s for interstate S1y–S1x electronic relaxation within a bound level structure of...

Isolated ultracold porphyrins in supersonic expansions. I. Free‐base tetraphenylporphyrin and Zn‐tetraphenylporphyrin

In this paper we report the results of an experimental study of the fluorescence excitation spectra and of the time‐resolved emission of Zn‐tetraphenylporphyrin (ZnTPP) and of free‐base tetraphenylporphyrin (H2TPP) seeded in pulsed supersonic expansions of He. We have studied the S0→S1 transition (the Q band) and the S0→S2 transition (the Soret, B band) of ZnTPP, as well as the S0→S1x transition (the Qx band), the S0→S1y transition (the Qy band), and the S0→S2x transition (the Bx band) of H2TPP. Information was obtained on the electronic energy levels, the vibrational level structure, the details of low‐frequency nuclear motion, and some characteristics of electronic relaxation.

Condensation limited cooling in supersonic expansions

Supersonic expansions of pure and seeded rare gases have been investigated experimentally, measuring the translational and rotational temperatures. The lowest achievable translational temperature in the jet depends on both gas properties as well as on experimental boundary conditions like nozzle shape and nozzle–skimmer distance. We show that there is a limit to the lowest temperature achieved, under practical conditions, set by condensation in the jet. A large cluster binding energy enhances the formation of clusters and they release their condensation energy into the beam. The spatial confinement of the jet extends to long distances, and is sensitive to the shape of the nozzle. The confined jet forms a narrow cone of high intensity, and results in increased collision probability and cluster formation.