Aviv Amirav

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.

Optical selection studies of electronic relaxation from the S1 state of jet‐cooled anthracene derivatives

In this paper we explore the energy dependence of the interstate electronic relaxation rates knr from the S1 manifold of anthracene and seven of its isotopic and chemical derivatives, which were inferred from quantum yield data. Absolute fluorescence quantum yields Y from groups of rotational states within the electronic origin S1(0) and from vibrational states were obtained over the excess energy Ev=0–3000 cm−1 above S1(0) by the simultaneous interrogation of the fluorescence excitation spectra and of the absorption spectra in seeded, pulsed, planar supersonic jets of Ar. Additional information was obtained from quantum yield data of van der Waals (vdW) complexes of these molecules with Ar. The fluorescence quantum yields from the S1(0) of anthracene, 9‐cyano‐anthracene, and 9,10‐dibromoanthracene were found to be independent of the rotational state, providing further evidence for the rotational independence of knr from a single doorway state. From the Y data of the electronic origins and from the Ev dep...

A direct sample introduction device for mass spectrometry studies and gas chromatography mass spectrometry analyses

A novel direct sample introduction (DSI) device is described, based on sample introduction in a disposable mini-vial (test tube) inside the existing gas chromatography (GC) injector. This DSI device effectively transforms a conventional GC injector, followed by a short capillary column, into a very cost-effective alternative to the standard direct insertion probe and enables a continuous sample flux at the mass spectrometer ion source for mass spectrometry (MS) and MS-MS studies. This same device also enables direct introduction of dirty samples for GC and GC-MS analysis. The dirty sample introduction is based on the thermal extraction of semi-volatile sample compounds inside the GC injector, while the non-volatile residue is retained in the sample vial that is disposed of after the analysis. The major capabilities of this new DSI device are demonstrated and its various features are discussed.

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...

Gas chromatography‐mass spectrometry with supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) with supersonic molecular beams (SMBs) (also named Supersonic GC-MS) is based on GC and MS interface with SMBs and on the electron ionization (EI) of vibrationally cold analytes in the SMBs (cold EI) in a fly-through ion source. This ion source is inherently inert and further characterized by fast response and vacuum background filtration capability. The same ion source offers three modes of ionization including cold EI, classical EI and cluster chemical ionization (CI). Cold EI, as a main mode, provides enhanced molecular ions combined with an effective library sample identification, which is supplemented and complemented by a powerful isotope abundance analysis method and software. The range of low-volatility and thermally labile compounds amenable for analysis is significantly increased owing to the use of the contact-free, fly-through ion source and the ability to lower sample elution temperatures through the use of high column carrier gas flow rates. Effective, fast GC-MS is enabled particularly owing to the possible use of high column flow rates and improved system selectivity in view of the enhancement of the molecular ion. This fast GC-MS with SMB can be further improved via the added selectivity of MS-MS, which by itself benefits from the enhancement of the molecular ion, the most suitable parent ion for MS-MS. Supersonic GC-MS is characterized by low limits of detection (LOD), and its sensitivity is superior to that of standard GC-MS, particularly for samples that are hard for analysis. The GC separation of the Supersonic GC-MS can be improved with pulsed flow modulation (PFM) GC x GC-MS. Electron ionization LC-MS with SMB can also be combined with the Supersonic GC-MS, with fast and easy switching between these two modes of operation.

Pesticide analysis with the pulsed-flame photometer detector and a direct sample introduction device.

New methods for fast, sensitive, and informative pesticide analysis in food products are described. These methods are based on sampling with a novel direct sample introduction device (DSI), gas chromatographic analysis, and pesticide detection with the pulsed flame photometric detector (PFPD). Sampling with the DSI is based on introduction of blended fruit or vegetable in a small glass vial that retains the harmful nonvolatile residue and is disposed of after the analysis. The DSI-GC-PFPD combination provides several new features that are demonstrated and discussed:  (a) Extract-free analysis is achieved with the DSI, which serves as an effective alternative to sample extraction and cleanup methods. (b) Faster analysis is achieved through the use of the DSI due to the reduction of the upper GC program temperature, since the low-volatility compounds are retained in the sample vial. (c) Relatively efficient and uniform DSI thermal extraction and PFPD detection can be achieved, allowing the use of internal standards for pesticide calibration. (d) Higher sensitivity is provided by the PFPD as well as through the use of the DSI for sampling larger volumes of extract solutions. (e) Sulfur interference is eliminated with the PFPD in its phosphorus-selective detection mode, using software exploiting differences in S and P delayed pulsed flame emission time dependence. (f) Sulfur pesticides are effectively analyzed by the PFPD in its sulfur-selective detection mode at the usual required levels and with less matrix interference than with NPD in the nitrogen mode. (g) Simultaneous sulfur and phosphorus pesticide analysis is demonstrated, including heteroatom identification and the provision of P and S atom ratio information in the analyzed pesticide.

Dynamics of trans-cis isomerization of stilbene in supersonic jets

Abstract The absorption spectra, the fluorescence excitation spectra and the relative fluorescence quantum yields of trans-stilbene in planar supersonic jets were determined for excess vibrational energies E V = 0–5000 cm −1 above the origin of S 1 . The non-radiative decay rates reveal an onset at E V ≈ 900 cm −1 , exhibit the erosion of vibrational photoselectivity for isomerization at E V = 900–5000 cm −1 and show a linear dependence on E V for E V = 1500–5000 cm −1 .

Aerodynamical acceleration and rotational-vibrational temperatures in seeded supersonic molecular beams

Abstract The time-of-flight (TOF) technique was used to study the aerodynamical acceleration in seeded supersonic molecular beams of heavy molecules and light seeding gas. We also studied the correlation between the degree of aerodynamic acceleration achieved, and rotational-vibrational temperatures as measured using the laser-induced fluorescence (LIF) technique. The velocity slip (difference) between helium and hydrogen carrier gases and iodine and aniline heavy molecules was determined in free-jet expansion by TOF measurements and compared with rotational temperatures measured by LIF. The helium translational temperature was found to be abnormally high and dependent on teh heavy-molecule concentration, even at concentration as low as 400 ppm. In the case of iodine it was found that the rotational degrees of freedom were equilibrated with the helium or hydrogen seeding gas translational and slip temperatures, although this temperature was more than an order of magnitude higher than theoretical predictions obtained for the pure-gas expansion. In aniline, the rotational temperature is found to be higher than the gas-dynamic temperature and rotational relaxation is incomplete. The heavy-molecule kinetic energy increases linearly with the light-gas pressure up to = 50% of its maximum available kinetic energy. The possibility of making accurate heavy-molecule kinetic-energy measurements using the LIF technique is discussed. It is claimed that the existence of velocity slip can largely effect theoretical calculations concerning vibrational and rotational relaxation in seeded supersonic beams.

External heavy atom effect on intramolecular intersystem crossing in a supersonic beam

Abstract We report the enhancement of the S 1 → T 1 intersystem crossing in van der Waals complexes of tetracene (T) with Kr and with Xe, which are studied by time and energy resolved laser spectroscopy in seeded supersonic beams. This external heavy atom effect is essentially due to single-pair TKr interactions.