The working group Molecular Spectroscopy was originally affiliated with the Institute of Physical Chemistry, Christian-Albrechts University in Kiel and since 1997 the Department of High-Frequency. It deals with the investigation of rotations spektra of isolated molecules in the millimeter and Submillimeter wave range:
The analysis of rotational spectra provides information on the molecular structure and dynamics. Investigations of vibrationally excited states provide additional information about rotation-vibration interactions.
Our spectrometer for receiving the rotational spectra is, like any absorption spectrometer, essentially consists of three parts: a radiation source, an absorption cell and a detector. These are major components, such as the following figure shows, by a variety of additional tools for control, stabilization, data collection, etc., adds:
Components of the Spectrometer.
The radiation sources, which of those are predominantly backward wave oscillators (Backward Wave Oscillator BWO) are used. In the millimeter wave range, we usually use a Ku-band BWO (Hewlett Packard 8690B Sweep Oscillator with insertion HP 8695A), which nominally delivers MW radiation from 12.4 to 18 GHz and with an amplifier (HP 8349B) and an active frequency multiplier (tripler HP 83556A HP 83558A or six-fold increase) that can be combined in Submillimeterwellenbereich Russian BWO (OB-30 OB-32) from FSUE RPC "Istok". For specific measurements, also Klystronröhren (OKI 20V10, 24V11 and 47V12, Varian AER 2102A-66 and AER 2103 B6 and Rodan KA 651) may be available to their respective output frequency, depending on requirements multiplied by passive point contact diodes. With these sources, the frequency range can vary from 12 to 120 and cover virtually complete 240-520 GHz.
The absorption cell consists of a cylindrical, double-sided discs with Teflon sealed glass tube of 4.20 m length and 10 cm in diameter. This is compared to the rotational spectroscopy brass waveguides with the advantage to remain on unstable substances, which decompose rapidly on metal surfaces, obtained in a glass cell under typical experimental conditions, a relatively long time in the measurement with necessary concentration.
For detection of the signals for measurements in very low frequencies (by this we mean the centimeter wave band) is commercially available microwave diodes are used, while in the Millimeter and submillimeter wave range, which is more important for us, a liquid helium cooled InSb hot electron bolometer (a "Putley detector ") is used. The cooling unit required for operation of helium liquefaction is a special model of the Canadian company Quantum Technology, the detector chip of the type QFI / X is the English manufacturer QMC.
The recording of the spectra is done using a computer-controlled frequency-stabilized radiation source. Here, the stabilization depending on the frequency range by one or two loops (phase lock loops, PLLs) is realized. When the Einkreisstabilisation only the Ku-band BWO with a acting as a local oscillator frequency decade (PTS 500) frequency-stabilized. The stabilized output signal is then amplified and then directly used for multiplication or as a measurement signal. For the measurements in the range of higher frequencies used dual circuit stabilization it is amplified, actively and then six times in a second control loop to stabilize the Russian BWO-tube (Istok OB-30 OB-32). (For more details see footnote.) The measurements after the so-called double modulation source (English source modulation). This method is based, as the German term suggests, on the principle of dual modulation of the radiation source: the slow modulation of the frequency sweep is a high-frequency modulation of small amplitude superimposed. For the slow frequency sweep (sweep frequency 5 Hz), which serves the recoating of the molecular signal, the frequency of the PTS is varied under computer control 500 in each of 1024 steps for the return sweep. The thus "swept" output signal, as described above for stabilization used the Ku-band BWO, which is determined by the phase-locked coupling to the frequency decade for his part "swept". This mechanism is necessary repeatedly in the second PLL. The output of the respective measurement source is now the 16-kHz sine wave signal of an RC-generator (HP 3310A) is superimposed. This second modulation is coupled to each of the construction of apparatus Synkriminator the first or the second control loop and is maintained during passage through the absorption cell and subsequent detection.
The detector signal is amplified and then to a working mode in 2f lock-in amplifier (Stanford Research Systems SR 510), where that is controlled by the same sine wave generator. The high-frequency modulation in conjunction with the 2f mode yields the second derivative of the Absoptionssignals. This one gets a better signal / noise ratio and sharper absorption lines. This one gets a better signal / noise ratio and sharper absorption lines.
The thus processed signal is finally passed through an A / D converter to the PC and stored in RAM as well visualized on the monitor. Through the use of the computer allows the measurement data of several cycles (each consisting of a return sweep) add up, hence the opportunity arises to improve the signal / noise ratio further.
Through the use of the computer allows the measurement data of several cycles (each consisting of a return sweep) add up, hence the opportunity arises to improve the signal / noise ratio further.
The measurements are usually carried out at room temperature (28.5 ° C) and conducted relatively low Measuring pressures in the range 1-3 Pa. In mmw and submmw range is the measurement accuracy in typical half widths of 75-500 kHz (half width at half maximum, HWHM) kHz, depending on line weight ± 5 to ± 25th
Further details regarding construction and operation of the spectrometer can, for example,  are removed.
Linear molecules have in the vibrational ground state is a very simple rotational spectrum: absorption lines occur in the spectrum always arises when the incident frequency of the expression ν = 2B(J + 1) – 4D(J + 1)3 + H(J + 1)3 [(J + 2)3 – J3] follows.
Here, B, D and H are molecule-specific constants, and J = 0, 1, 3, ... is the rotational quantum number. A molecule absorbs radiation of ? by the expression above defined frequency, it goes from one described by the rotational quantum number J state to the next level, above-described J + 1.
Because the rotational constant B typically has a value of several GHz, while the centrifugal distortion D or H in general in the Hz or mHz range, is the pattern that is visible in the spectrum, mainly from the first term of the above frequency determined expression. The rotational spectrum thus consists of individual absorption lines, be the distances of each other about 2B.
The transition to the excited vibrational states, the spectra are arbitrarily complicated:
An N-atom-linear molecule has 3N - 5 vibrational degrees of freedom, the spread (vertical vibrations to the molecular axis) to N-1 stretching vibrations (vibrations within the molecular axis) and N-2 doubly degenerate bending vibrations. In general, the levels of the singly excited bending vibrations are energetically so low that their populations are large enough for a rotationsspektroskopische investigation, while stretching excited states located at higher energy and therefore are accessible only in part.
In general, the levels of the singly excited bending vibrations are energetically so low that their populations are large enough for a rotationsspektroskopische investigation, while stretching excited states located at higher energy and therefore are accessible only in part. In addition to the singly excited vibrational states (in which the vibrational quantum number ? has the value 1), there are higher states (overtones with ?> 1, and combination vibrations are excited in which two or more oscillations at the same time).
For the description of rotational transitions within these higher excited vibrational states in addition to the rotational quantum number J must further quantum numbers are considered and the expressions for the absorption spectrum includes not only B, D and H, a plurality of vibration-rotation interaction constants. A rotational transition J + 1 ← J leads in these cases not to a single absorption line, but it splits into a doublet, triplet, quartet, or a higher multiplet:
Typical Quartet of a Combination Vibrational State
(HCCNC, Vibrational State (υ4υ5υ6υ7) = (0011), transition J = 5 ← 4).
Prof. em. Dr. Antonio GuarnieriDr. Aiko Huckauf
Technische Fakultät der Christian-Albrechts-Universität zu Kiel
– Lehrstuhl für Hochfrequenztechnik –
D – 24143 Kiel
Fon +49 (0)431 880-6155Fax +49 (0)431 880-6152