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Accueil > EN > Research Areas > Pure and applied spectroscopy > Pure and applied spectroscopy > Equipments

Fourier Transform Microwave Spectrometer

par Manuel GOUBET, Pascal DREAN, Thérèse HUET - publié le , mis à jour le


A Fourier Transform MicroWave (FTMW) spectrometer permits to record the pure rotational spectrum of a sample in the gas phase cooled by adiabatic expansion (supersonic jet). Unique in France, 2 Balle-Flygare type devices are operated by the team for spectroscopic analyses in the 2-20 GHz spectral range. These spectrometers, characterized by a very high resolution and high sensitivity, allow the observation of any molecular system provided it has a permanent electric dipole moment higher than 0.1 Debye (centro-symmetric systems cannot be observed).

Advantages of the set-up

For a short overview of the interest of a supersonic jet in spectroscopy, see the Jet-AILES apparatus

With the support of quantum chemistry calculations, recording and modelling the pure rotational spectrum of a molecular system lead to the unambiguous identification of its structure. Indeed, the observation of the rotation of a molecular system around its inertial axes permits to obtain its rotational constants, which correspond to a unique geometry. The molecular structure then gives access to the physico-chemical properties of the studied system. It is also essential to be able to model the spectrum of a given molecule for identification during observational campaigns (atmospheric or interstellar), eventually. Moreover, the conformational relaxation offered by the jet allows identification of the most stable conformations. Indeed, despite the increase in the number of possible structures with the size of the molecule, the nature does retains only a few of them, the most stables energetically. Finally, the very high resolution of the spectrometer gives access to hyperfin structures such as quadrupolar, spin-spin or spin rotation couplings.

Description of the set-up

A short and intense microwave pulse creates a macroscopic polarization of the molecular gas when this pulse is resonant with a rotational transition. When the electromagnetic field is cut-off, the molecules emit a free induction decay signal (time domain) oscillating at the resonance frequency. The Fourier transform of this signal gives the amplitude spectrum (frequency domain).

  • nutation-precession process

    Consider a gas, composed of n0 molecules having a permanent dipole moment μ, with a macroscopic polarization P. A short and intense microwave pulse, supposed resonant, with amplitude E0 and pulsation ω0, is absorbed by n molecules.

    • The excitation sequence :
      • Then exists a nutation frequency Ω (Rabi frequency) of the dipoles around an axis defined by the electric field of the exciting microwave pulse : this is the optical nutation.

        \left\{\begin{array}{rcl} n&=&n_0cos( \Omega t)\\P(t)&=&-i \mu n_0sin( \Omega t)\\ \Omega &=& \mu E_0\over\hbar\\\end{array}\right

      • Then occurs a macroscopic polarization of the gas which is maximum if sint) = 1 : this is the π/2 condition.

    • The detection sequence :
      • The macroscopic polarization of the gas decreases exponentially, oscillating at the resonance frequency (ω0) of the gas : this is the optical precession.
         P(t)=n_0 \mu e^{-{t\over T_2}} \cos(\omega_0 t) with T2 the relaxation time

      • Detection of the signal  S(t) \propto P(t)  : a variation of the electric field with a pulsation ω0 is associated to the oscillation of P(t). It is detected by a simple antenna.

  • The Fabry-Pérot resonator
    • A signal amplifier.

      The signals are amplified using a quasi-confocal Fabry-Pérot cavity. The microwave pulse is sent via an L-shaped antenna, located in the axis of the cavity. Then, the signal emitted by molecules is detected by this same antenna. For each frequency, the length of the cavity is adjusted (resonant) by moving one of the 2 mirrors using a micrometric step-motor.

    • Coaxial arrangement : Doppler splitting.

      In order to maximize the signals, the supersonic jet and the cavity axis are coaxial. Since the jet is unidirectional and the electromagnetic wave propagates per round-trip in the cavity, molecular transitions are observed as Doppler doublets (splitting of several tens of kHz depending on the carrier gas and the frequency). The frequency of the transition is then measured by averaging the frequency of the two Doppler components.

  • The software

    The experimental sequence, from the gas injection (solenoid valve used at a repetition rate around 1 Hz) to the acquisition of the spectrum (digitized at a repetition rate of 120 MHz), is fully controlled by a homemade software allowing on-the-fly adjustment of the instrumental parameters. 2 acquisition modes are available :

    • The "low resolution" mode allows an automatic scan within a chosen frequency range. The molecular signals are then pointed out with an accuracy of a few hundreds of kHz.

    • The "high resolution" mode allows to accumulate acquisitions at a given (fixed) frequency. The signal has a better signal-to-noise ratio and is pointed out with an accuracy of a few kHz.

Recent publications

  • Structural and Dynamic Properties of a Hydrogen Bond from the Study of the CH3Cl-HCl Complex and Isotopic Species,
    M. Goubet, P. Asselin, P. Soulard and B. Madebène, J. Phys. Chem. A
    117, 12569 (2013)

  • Conformational relaxation of S-(+)-carvone and R-(+)-limonene studied by microwave Fourier transform spectroscopy and quantum chemical calculations, J.-R. Aviles Moreno, T. R. Huet, J. J. Lopez Gonzalez, Struct. Chem., DOI 10.1007/s11224-012-0142-8 (2013)

  • Magnetic hyperfine coupling of a methyl group undergoing internal rotation : a case study of methyl formate, M. Tudorie, L. H. Coudert, T. R. Huet, D. Jegouso, G. Sedes, J. Chem. Phys. 134 074314 (2011)