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Paul Scherrer Institut PSI Molecular Dynamics

Paul Scherrer Institut
5232 Villigen PSI, Schweiz/Switzerland
Tel. +41 56 310 21 11
Fax. +41 56 310 21 99

E-Mail: thomas.gerber@psi.ch

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Identification of a new low energy 1u state in dicopper with resonant four-wave mixing

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A' is identified with T0 = 20 100.4090(16) cm-1 (63Cu2). Rotational analysis of the A'-X (0,0) and (1,0) transitions leads to the assignment of A' 1u. Ab initio calculations present the first theoretical description of the low energy states of the copper dimer in Hund's case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state.
In parallel, state-of-the-art high-level ab initio were performed considering the low energy states in Hund's case (c) terms. The results prove the assertion that spin-orbit coupling is required to account for the number of observable states.

Four-wave mixing spectra of Cu2. (a) depicts a DFWM spectrum of the (1,0) B1Σ+u (0+u)- X 1Σ+g band. Inverted, a simulated spectrum for 63Cu2 and 63Cu65Cu is shown in green and red, respectively. Simplified TC-RFWM spectra are obtained by intermediate labeling of specific rotational transitions (b)-(d).

Shedding light on a dark state: The energetically lowest high-spin state of C2

The Swan band emission between 400 and 700 nm is a prominent feature in all carbon-containing flames. The intense d 3Πg-a 3Πu electronic transition is widely used to detect the molecule in combustion and astronomy studies to characterize and test chemical mechanisms. The quantitative interpretation of spectra requires precise molecular constants for the computation of the complex molecular spectra of C2.

In this work we report on the deperturbation of the d 3Πg, v=6 state by double-resonant four-wave mixing. The high sensitivity and dynamic range of the method allows observation of 'extra lines'. These weak spectral features originate from nearby-lying, optically dark states that gain transition strength through the perturbation process. The study unveils the presence of the energetically lowest high-spin state (5Πg) in the vicinity of the g, v=6 state and unravels major issues of the so-called high-pressure bands of C2. The anomalous nonthermal emission initially observed in 1910 and later in numerous experimental environments are rationalized by taking into account 'gateway' states, i.e. rotational levels of the g, v=6 state that exhibit significant quintet character through which all population flows from one electronic state to the other.

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First detection of an anion by DFWM and TC-RFWM

Molecular anions are relevant in astronomy, in the upper atmosphere, in laboratory discharges and in combustion. Actually, anions are present in most combustion system and are assumed to play an important role in reactions forming pollutants like soot and aerosols from aircraft-engines. In a recent work, Warnatz and coworkers included negative ions in the chemical reaction mechanism of the modeling of a fuel-lean methane-oxygen flame and emphasized the relevance of anions. In particular, detailed reaction mechanisms involving negatively charged species are required for the calculations of the electrical conductivity which is essential for technical applications.
Generally it is difficult to generate anions in sufficient abundance for spectroscopic studies. In fact, negative ions are far less abundant in most of the experimental environments than neutrals or cations. Often, anions are generated in a plasma where numerous species coexist whose spectra may overlap. Therefore, highly sensitive methods are required that are suitable for low-density environments. In addition, the identification and characterization of specific ions demands techniques exhibiting a substantial selectivity in order to disentangle the spectral features. An additonal challenge for spectroscopic investigations of anions is the binding energy of the excess electron that is usually small such that excited electronic states cannot exist. In this work, we demonstrate for the first time to the best of our knowledge that nonlinear four-wave mixing is sufficiently sensitive for the detection of anions in a discharged free-jet. Both degenerate and four-wave mixing spectra of C2- are measured with an excellent signal to noise ratio. By comparison with cavity ring-down measurements under similar conditions, a detection limit for the carbon dimer anion of 107 anions/cm3 is estimated.

The result suggests convincingly that nonlinear four-wave mixing spectroscopy is applicable to study numerous neutral, cationic and anionic radicals that are produced in the controlled environment of a molecular beam by applying a discharged free-jet expansion.
The figure shows a DFWM spectrum of the C2 anion (upper trace). By applying two-color resonant four-wave mixing, the spectrum is simplified substantially. By tuning the PUMP laser to the (1,0) R1(6.5) transition and scanning the PROBE in the wavelength region of the sequence bands with Δν=0 four transitions are observed only: Two UP type transitions from the common J=6.5 level and two SEP transitions to J=7.5.

Deperturbation of the C2 Swan bands (d 3Πg - a 3Πu) by TC-RFWM

Numerous studies of the C2 radical reflect its importance in combustion, plasma and astro-chemistry. Recent LIF and polarization spectroscopy measurements have been performed to measure the absolute abundance in flames and plasma. However, a quantitative reduction of the measured data requires accurate molecular constants. These data are essential to compute the complex spectra exhibiting many overlapping transitions. In particular, an accurate simulation of the C2 spectra is essential in a combustion or plasma environment where spectral features from multiple components are present within the extended wavelength region of the Swan band.
The determination of accurate molecular constants for the Swan bands is, however, difficult owing to the heavy line congestion and the numerous perturbations due to interactions with neighboring electronic states. In this work, we take advantage of the high sensitivity of four-wave mixing spectroscopy to investigate rotational perturbations occuring in the d 3Πg-a 3Πu electronic system of C2 in a discharged free-jet. We use intermediate level labeling by the double-resonance capabilites of the method to simplify the congested spectra for unambiguous assignments of perturbed and perturbing transitions. The investigations yield coupling constants that allow improved spectral simulations of C2 Swan band.
A DFWM of a heavily perturbed region of the C2 Swan band is shown (black trace). Two TC-RFWM scans by labeling the F2(8) and F1(14) rotational states, respectively, in the a 3Πu ground electronic state are shown in the upper trace. Unambigously, the perturbed R2(8) and R1(14) transitions are observed. In addition, transitions labeled by R2(8)* and R1(14)* are observed due to the "perturbing" levels of the B 1Δg. The lowest two traces (red and green) show the spectral simulation which accounts for the d 3Πg - B 1Δg interaction.

Rotationally resolved double-resonance spectroscopy of low-lying vibrational ground-state levels of HC4S

HCnS chains (n ≤ 2–8) have been subject to several studies by different spectroscopic techniques due to the relatively high abundance of sulfur in the interstellar medium and the identification of sulfur containing carbon chains by radio astronomy. To develop the application of the four-wave mixing technique to radicals produced in a slit-jet discharge, we performed an investigation on 2π3/2 symmetry rovibronic levels in the range of 549 and 580 cm-1 above the electronic ground state of HC4S. The levels are observed by applying two-color resonant four-wave mixing in a pulsed-discharge slit-jet expansion. Excitation is achieved via selected rotational transitions in the origin band 000 of the à 2π3/2 - X̃ 2π3/2 electronic transition. By combining the new measurements with recent spectroscopic data for the à 2π3/2 state, the origin band and the rotational constants for the 51 (CS stretching) and 8191 (bending) modes are determined.
TC-RFWM SEP spectrum of HC4S. Several R-transitions in the bandhead of the 000 Ã 2π3/2 - X̃ 2π3/2 transition are simulantaneously excited by tuning the PUMP laser to 19982.58 cm-1. By scanning the DUMP laser an overview of the ground state vibrational levels in the vicinity of 51 are observed.

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