CHAPTER ONE INTRODUCTION
CHAPTER ONE
INTRODUCTION
The region of the electromagnetic spectrum between 300GHz and 3THz is
loosely defined as the Far Infrared (FIR). Research in this area only began in earnest
around 25 years ago, limited by a lack of coherent sources and suitable detectors.
Advances were driven by observations of FIR emission spectra from astronomical and
atmospheric sources, extending traditional Microwave and Infrared (IR) techniques to
the FIR. A wealth of chemical and physical information is obtained from FIR
spectroscopy of gases, liquids, and solids. Photon energies are around five times lower
than Hydrogen Bond energies (0.1 to 1kJmol-1) making FIR spectroscopy an ideal probe
of inter- or intra- molecular displacements below chemical decomposition limits.
Molecular gas phase work in this region focuses on Van der Waals and Hydrogen bond
vibrations, pure rotational transitions in light molecules, and torsional rotations or ring
puckering in non-rigid molecules.
The emission maximum for a blackbody radiating between 3 and 30K occurs in
the FIR region, shifting further into the IR as the temperature increases (figure 1.1). FIR
absorption spectra are only observed if the absorbed power is greater than the
background level, or the incident FIR radiation field is enhanced, as in laser spectroscopy
[1]. For a transition in a gas in thermal equilibrium, the total power absorbed per cm3, Pij,
depends upon the relative populations of the transition energy levels (governed by the
Boltzman distribution) the absorption volume of the sample V, the total incident power
at the transition frequency, Po(), and the absorption cross section ij [1]:
 h ij
N
g i exp
Z
 kT

V

(1.1)

where N is the total number of molecules in the sample and Z is the partition function.
Pij  Po ( o ) ij
The absorption cross section per molecule is between 10 and 105 times higher in the FIR
than in neighbouring regions (IR and microwave respectively). Spectral line intensities
1
CHAPTER ONE INTRODUCTION
are also dependent upon the transition probability, |Rij|2, given by the square of the
transition dipole moment [2]:
R
ij
   *   d
i ij j
(1.2)
where the dipole moment, ij, is determined by the type of transition under consideration.
A molecule must possess a permanent electric dipole moment to observe its pure
rotational spectrum. As a consequence of these large transition probabilities, pure
rotational FIR spectra are more intense than vibrational IR spectra. The combination of
large absorption cross sections and large transition probabilities makes FIR spectroscopy
a very sensitive molecular probe provided the technological limitations of working in
this region are overcome.
Radiation Density
(arb units)
T=300K
FIR
11
1.77x10 Hz
(1.6mm)
T=3K
1E8
1E9
1E10
13
1.75x10 Hz
(17.1m)
1E11
1E12
1E13
1E14
Frequency (Hz)
Figure 1.1: Comparison of Plank’s Blackbody Radiation Curves at 3 and 300K and
the FIR region of the electromagnetic spectrum. (Curve at 3K is shown x105)
The remainder of this chapter gives a historical background and overview of gas
phase FIR spectroscopy, highlighting tunable FIR techniques in particular. The following
section describes the development of tunable FIR spectrometers in parallel to the many
technological advances in this field. The current status of these techniques is also
mentioned. This thesis is based on high resolution, Tunable Far Infrared (TuFIR) Laser
Spectroscopy of free radicals and stable species. Therefore, TuFIR generation by nonlinear mixing is discussed in greater detail, from its inception in the 1980’s to the present
day. There are currently eight groups that are using TuFIR Spectroscopy worldwide: the
Cambridge spectrometer is unique in the U.K. The previous work of these groups is
2
CHAPTER ONE INTRODUCTION
summarised. The final section outlines the Cambridge TuFIR spectrometer, giving an
overview of the rest of this thesis.
1.1 Gas Phase FIR Spectroscopy
The physicist Rubens [3] published the earliest papers on the FIR region
commencing in 1893. He used ‘reststrahlen’ to investigate the electromagnetic properties
of materials and studied the effects of gratings and meshes on the polarisation of FIR
radiation. Together with Bayer, Rubens discovered that a mercury arc lamp could be
used as an incoherent, polychromatic FIR source [4]. This remained the main source of
FIR radiation until the 1950’s! Czerny recorded the first FIR spectra in 1925 when he
observed the pure rotational spectrum of HCl between 30-120m [5]. A range of other
molecules were then investigated including NH3, PH3, and H20 [6,7]: the first review of
FIR spectroscopy was published in 1938 [8]. These early experiments all used spark
generators or mercury arc lamps as sources and thermocouple or bolometer detectors,
operating at room temperature.
1.1.1 Broadband FIR Spectroscopy
FIR spectroscopy was initially performed using dispersive elements such as
prisms or gratings. Prisms were constructed from alkali halide materials that strongly
absorbed FIR radiation above about 30m, e.g. NaCl, KBr. The exact cut-off frequency
depended upon the ‘reststrahlen’ absorption region, which shifted to lower frequencies
for larger and heavier atoms [9]. Much of this early work would now be defined as midIR as opposed to FIR spectroscopy. Grating monochromators were exclusively used
above 50m. The energy throughput of this type of spectrometer depended on the
emissivity of the source, cross sectional area of the grating, the slit length and bandwidth,
and the transmission efficiency of the instrument [10]. The instruments were designed
with large apertures, long slits and very large gratings, (900cm2) [9] to maximise this
throughput energy and optimise their resolving power (/d) [11]. Usually an echellette
diffraction grating with a blaze angle between 20o and 30o was used. Two particular
optical arrangements emerged, developed by Czerny-Turner [12] and Ebert-Fastie [13,
14], to reduce the beam aberrations. In both cases, the grating itself was rotated,
reflecting incident radiation into the first order fringe. Grating spectrometers were
3
CHAPTER ONE INTRODUCTION
commercially available, operating from 25 to 330m with resolving powers around
14GHz.
These spectrometers had two major disadvantages:
1. a broadband FIR source was used, so for a particular grating alignment at
wavelength , radiation of wavelength /n was also incident on the grating
and reflected into the nth order fringe. If no suitable filters were available to
remove these wavelengths anomalous absorptions would arise in the
spectrum,
2. at longer wavelengths larger slit widths were required to maintain the
resolving power of the instrument. Increasing the slit dimensions increased
the beam aberrations and meant that the slit itself had a larger aperture than
the FIR source or detector. There was therefore no advantage in using larger
slit dimensions and the resolution was reduced at long wavelengths.
Despite these shortcomings a number of molecules were studied using grating
spectrometers including CO, NO, CH3D, and HCN [9,11]. Further examples can be
found in review papers on the subject [15, 16].
Fourier Transform instruments such as Michelson or Lamellar grating
interferometers gradually replaced dispersive spectrometers. All interferometers operated
in essentially the same way: FIR radiation from a broadband source was collimated into
a parallel beam and modulated, usually by a chopper. The beam was then spatially
divided into two equally intense beams using a beamsplitter in the Michelson case and a
Lamellar grating (interlocking mirror segments) in the Lamellar instrument. A path
difference was introduced between these two beams before they were recombined. The
beam then passed through the sample and was recorded at the detector. The output
intensity, I(x), varied according to the path difference between the two beams, x, and the
sample absorption, S(), [11]:

I ( x) 
 S ( ) exp 2ix dx
(1.3)

The Fourier transform of this ‘interferogram’ gave the absorption spectrum [11]:
S ( ) 

 I ( x ) exp2ix d
(1.4)

4
CHAPTER ONE INTRODUCTION
The Fourier transform was evaluated using numerical methods, and latterly computers.
Recently these types of calculations have been simplified by high-powered PC’s and fast
Fourier Transform techniques [17].
There were many advantages to Fourier Transform spectroscopy [11]:
1. the throughput of these instruments was significantly larger than the grating
instruments since the whole spectral range was simultaneously incident on the
detector. Consequently, for a spectrum of n points, recorded in a time t, the
signal to noise ratio (S:N ratio) was proportional to t as opposed to(t/n) in a
grating instrument. This is known as Fellgett’s advantage and is particularly
important in the FIR where S:N ratios are limited by weak radiation sources
and detector sensitivities,
2. the interferogram could be recorded in a much shorter time period than the
dispersive spectrum, whilst maintaining the resolution and S:N ratio. This
time advantage is known as Jacquinot’s advantage,
3. filtration was much simpler in interferometric systems, where only short
wavelength IR had to be removed to prevent ‘aliasing’. A wider spectral
range could therefore be recorded using interferometric instruments.
Consequently the sensitivity of interferometric systems was around two orders of
magnitude greater than the grating systems.
The first FIR Fourier Transform spectrometers (FIR-FTS) were built in the
1950’s at the National Physics Laboratory (NPL) in the U.K. [18]. Despite the fact that
Lamellar gratings were more efficient than beamsplitters, Michelson interferometers
were most popular as they were easier to construct. A number of technological advances
improved the sensitivity of FIR-FTS [17]. Two of the most significant developments
were the introduction of the Golay detector [19] and capacitative grid filters [20]. These
advances were incorporated into commercially available instruments [18] which were
used to measure the pure rotational spectra of isotopic species of HCN, N2O and CO
between 25GHz and 3THz at a resolution of 500MHz [11]. These measurements were
established as early calibration standards in the FIR.
5
CHAPTER ONE INTRODUCTION
The beamsplitters in these early FIR-FTS were constructed from a stretched sheet
of Melinex. The transmission and reflection coefficients of this plastic varied
significantly with wavelength and sheet thickness. Consequently ‘gaps’ appeared in the
absorption spectrum where data was missing from the interferogram. In 1969 Martin and
Pulpett designed a novel ‘polarisation’ interferometer in which the Mylar beamsplitter
was replaced by a set of finely spaced parallel metal wires [21]. The interferograms
could be collected across frequencies ranging from almost DC to c/2d, where d was the
spacing between the wires. With this type of instrument it was also possible to record the
sample and background signals simultaneously so a true zero level could be obtained in
the interferogram. This type of interferometer is used in modern FIR-FTS. The effective
resolution is doubled by using a folded optical arrangement, and the S:N ratio is
improved by cryogenically cooling the detector, optics, and interferometric chamber.
FIR-FTS are widely used in atmospheric remote sensing [22] and FIR astronomy
[23, 24]. This technique has also been applied to low resolution laboratory studies of
reactive intermediates or large molecules (>6 atoms). Dinelli et al recently measured the
isotopic ratio of HDO to H2O in the stratosphere using FIR-FTS. Their instrument
operated between 1 and 2.5THz, with a spectral resolution of 75MHz [25]. Morino et al
detected NH, NH2, NHD, and ND2 radicals using FIR-FTS and resolved the fine
structure in the spectrum [26]. The same group observed the SH radical and were able to
determine many of its ground-state constants [27]. Winnewisser et al have used FIR-FTS
to study a number of large, stable molecules at relatively high resolution (45MHz) e.g.
HC6H [28]. Moller and co-workers at Brookhaven National Laboratory, U.S.A, used a
FIR synchrotron source for FIR-FTS. The increased source brightness improved the S:N
ratio in the spectrum [29]. This type of system has been used to record the spectrum of
IBr with a resolution of 35MHz [30]: this is currently the highest spectral resolution
achieved in a FIR-FTS instrument.
1.1.2 Microwave Techniques in FIR Spectroscopy
The first coherent FIR sources were developed by Gordy and co-workers at Duke
University in the early 1950’s [31]. The output from a millimetre wave source was
passed through a non-linear crystal generating a number of higher frequency harmonics
in the FIR. These harmonics were transmitted from the crystal using a waveguide, whose
cut-off frequency was designed to filter out the intense millimetre-waves. If the
6
CHAPTER ONE INTRODUCTION
millimetre source was widely tunable the harmonics overlapped, producing a
continuously tunable, CW, narrow-band source of FIR radiation. The power output of
these crystal harmonic generators was limited by the energy of the millimetre wave
source, and decreased rapidly at higher harmonics, (a few W by the 4th harmonic). King
and Gordy combined these sources with a Si crystal detector to measure the pure
rotational spectra of OCS, CH3F, and H2O around 300GHz [32]. They determined the
centrifugal distortion constants of these molecules for the first time. Transition
frequencies were reported with a few hundred Hz accuracy, and the spectral resolution
was around 200kHz. By 1956, Gordy had extended this harmonic generation beyond
400GHz [33]. Higher order harmonics with increased output powers were achieved when
GaAs mixers replaced Si crystals. Helminger et al re-measured the OCS spectrum,
extending the data to 800GHz [34], using a GaAs crystal and a Putley InSb detector [35].
The spectrometer sensitivity was increased by more than one order of magnitude from
the original experiments. These spectrometers were used to detect the first FIR radial
spectra, e.g. CN, H2S [9]. In comparison to FIR-FTS, multiplier techniques produced
much higher resolution spectra, with better S:N ratios.
Subsequently, harmonic generation has been extended beyond 1THz using higher
frequency sources, improved mixers, and more sensitive, cryogenically cooled detectors
[36]. However, the optimum sensitivity range of such instruments is still around 300650GHz. Modern multiplication spectrometers fit into two categories:
1. broadband multipliers operating at low power,
2. narrow-band and finely tunable multipliers operating with much higher
efficiencies.
The broadband multipliers cover a very wide range of frequencies, typically 200900GHz but the power output varies greatly in different harmonics, particularly at low
orders (2nd, 3rd) or very high orders (>10th) [37]. Therefore this type of generator is not
well suited to FIR spectroscopy. Narrow-band multipliers are optimised to operate at a
fixed frequency, with a minimal tuning range (around 50GHz). Below 650GHz, output
powers in excess of 100mW are easily attainable [38]. These multipliers have been
widely used in remote sensing applications and ground or satellite based ‘radio’
astronomy to observe specific and well-characterised absorption lines. For more general
spectroscopic investigations, a number of interchangeable multipliers are required. The
7
CHAPTER ONE INTRODUCTION
main disadvantage of these systems is their cost! Recently Ziurys et al designed a FIR
harmonic generation spectrometer, operating between 65 and 550GHz, with the potential
to reach 800GHz. They observed the pure rotational spectra of a number of transient
molecules, including CaOH, MgOH, CaH, MgF, and BaOH, with a spectral resolution
between 200kHz and 1MHz [39]. This was sufficient to resolve both the isotopic
splittings and nuclear hyperfine transitions.
Backward Wave Oscillators (BWO’s) were originally developed as millimetre
wave sources [40]. They are constructed from large periodic metal cavities, similar to
simple waveguides. A 4-8kV collimated electron beam passes through the cavity,
inducing currents in the metal surface. These currents generate an electromagnetic wave,
which counter-propagates along the cavity, i.e. kinetic energy from the electron beam is
transferred into the electromagnetic wave (backward wave) [41]. The cavity dimensions
define the propagation constant and the transverse and longitudinal modes of the
electromagnetic wave: the frequency depends upon the mean velocity of the electron
beam. Since a single cavity can sustain a number of modes, the BWO is electrically
tunable over a large bandwidth, e.g. 30GHz. The output power of such devices in the
10GHz region was originally a few Watts [40].
Higher frequency BWO’s were developed as an alternative to harmonic
generation FIR sources. Krupnov and his colleagues at the Nizhnii Novgorod Microwave
Spectroscopy Laboratory in Russia pioneered this work [42]. They combined freerunning BWO’s with acoustic detectors, and measured the spectra of N2O, SO2, H2O,
and HCOOH up to 1THz. The reported frequency error on each transition was a few
hundred kHz, and the spectral resolution was around 1MHz. The output power of these
BWO’s varied from 10mW at 300GHz to 1mW at 1THz, at operating voltages of 1.5 and
5kV respectively. Recently, Krupnov’s group has collaborated extensively with groups
in the U.S.A and Germany. They have observed spectra from molecular complexes e.g.
(HCl)2, H2O-HF, small molecules e.g. AsH3, H2Se, and ions e.g.H2D [43]. A broadband
scanning THz spectrometer was developed in collaboration with Winnewisser’s group in
Cologne, operating between 150GHz and 1THz. The spectrometer was used to record the
spectra of transient molecules such as HSSH, DSSH, and DSSD up to 1THz, with a
Doppler limited resolution around 400kHz [44]. The Nizhnii Novagorod Laboratories
have commercially developed a number of millimetre-wave and submillimetre-wave
8
CHAPTER ONE INTRODUCTION
synthesisers, based on BWO technology. These synthesisers output FIR radiation to
500GHz and have been used to measure the pure rotational spectrum of OCCCS [45].
The BWO acoustic spectrometer is currently used up to 1.524THz for gas analysis work,
with Doppler limited resolution (300kHz) [43]. Petkie et al developed a rapid scanning
FIR spectrometer, based on a 240-375GHz BWO, for the same purpose. To illustrate its
plausibility they re-recorded the pure rotational spectrum of HNO3 in the =7 vibrational
state [46]. Such systems are viable alternatives to FIR-FTS.
The major disadvantages of BWO’s are that they are very expensive, due to the
severe machining tolerances on the BWO cavities, require large stabilising magnets, use
very high voltages, and are only available from a single supplier (ISTOK Research and
production Company, Fryazino, Moscow). Although a single BWO covers a very broad
spectral range (typically a few hundred GHz) the relatively short lifetime of each BWO,
and the necessity to own ten or more such devices for a full coverage of the FIR
precludes most FIR spectroscopists from using them.
1.1.3 FIR Laser Spectroscopy
The first FIR laser was an electrically discharged Neon laser developed in 1963.
Faust et al [47] then Patel et al [48] generated a few nW of pulsed FIR radiation on a
number of discrete wavelengths up to 132.8m. The following year Crocker et al
reported that FIR laser action could be achieved in H2O-vapour pumped by a pulsed
electrical discharge [49]. The output power was only a few W per pulse, but a number
of wavelengths were observed up to 774m. In 1966, Müller and Flesher demonstrated
that a H20-vapour laser could be operated in continuous wave (CW) mode [50]. Laser
action was also observed from D2O and H218O isotopes [51]. Gebbie et al were the first
to produce stimulated emission in HCN [52]. They achieved output powers up to
200mW over a wavelength range from 40m to 337m. Similar laser action was also
observed in H2S, OCS, and SO2 [50].
The tuning range of these FIR electrical discharge lasers was limited to around
5MHz, i.e. the full width of the gain profile. In practice, the laser output was fine-tuned
by altering the cavity length. Only certain discrete line frequencies were obtained from
each gas so spectroscopic work with these lasers was limited studies of molecular
9
CHAPTER ONE INTRODUCTION
transitions close to the laser output frequency. Duxbury and Burroughs investigated
CF2=CH2 and D2O this way [53]. Stark spectroscopy was used to investigate molecules
possessing permanent electrical dipole moments e.g. HD3, NH3. Evenson et al observed
NO2, NO, CH [54] and OH [55] radicals using a Zeeman modulated cell inside the cavity
of an HCN laser [56]. These experiments were the forerunners to FIR Laser Magnetic
Resonance (LMR).
The optically pumped FIR laser had a more significant impact on FIR
spectroscopy. Chang and Bridges first reported optically pumped laser action in 1970
[57]. They obtained pulsed emission at 452, 496 and 541m from CH3F in a Fabry-Perot
cavity, pumped by a CO2 laser. CW operation was demonstrated later that same year
[58]. To date over 4000 optically pumped FIR laser transitions have been reported from a
variety of gases compared with only 300 electrically pumped lines. Reference [59]
clearly documents all these CW FIR laser lines. The output power of these lasers ranges
from a few W to hundreds of mW depending on the lasing gas and pump frequency.
Despite their wider frequency coverage, optically pumped FIR lasers were still
only discretely line tunable and therefore no more suitable for broadband spectroscopic
applications than FIR electric discharge lasers. Instead, two techniques were developed
whereby the molecular energy levels were ‘tuned’ into resonance with the laser
frequencies, using an external electric field (laser electric resonance, LER), or an
external magnetic field (laser magnetic resonance, LMR). Provided that a molecule
possessed a permanent electric or magnetic dipole moment its spectra could be recorded
with Doppler limited resolution and very high sensitivity, since the absorption cells could
be placed within the laser cavity. LMR was used more extensively than LER and was
capable of detecting 106-107molec.cm-3 [38]. There were three main drawbacks to these
techniques:
1. the external field lifted the degeneracy of the MJ-levels complicating the
spectra,
2. to determine the exact zero field transition frequencies within a few MHz,
each spectrum had to be recorded using a number of different laser lines, so
the density of lasing transitions in the region of interest had to be high,
3. LMR could only detect open-shell species.
10
CHAPTER ONE INTRODUCTION
For these reasons the number of molecules that could realistically be studied using LMR
was limited. FIR-LMR was used to study radicals and ions of astronomical and
atmospheric importance: this work is summarised in two reviews by Evenson [60] and
Saykally [61]. More recently FIR-LMR was used to study HS2 [62], and AsH [63] in the
gas phase for the first time. Nolte et al have observed CHF2 radicals using FIR-LMR
[64], and Brown et al have observed fine structure transitions in the F+ atom [65]. This
year, Hubers et al built a new FIR-LMR spectrometer replacing the usual electromagnet
with a permanent magnet [66]. Despite its recent decline in popularity, FIR-LMR
remains the most sensitive FIR spectroscopic technique to date.
To overcome the limitations of LMR and LER spectroscopy, techniques were
developed to generate discrete, step tunable FIR ‘sidebands’ from non-linear mixing of
laser radiation with a tunable source. In the earliest experiments, Zernike et al mixed two
outputs from a Nd-glass laser in a quartz crystal, producing pulsed FIR radiation at
100m [67]. The conversion efficiency was very poor and they only obtained a few nW
of FIR power. ‘Reststrahlen’ effects in the crystal limited the FIR output frequency.
Yajima et al overcame this problem by using ZnTe semiconductor crystals. They
generated pulses of 350m radiation from the difference frequency between the R1 and
R2 lines of two ruby lasers [68]. The FIR output could be tuned between 300GHz and
1.5THz by heating or cooling one or both of the ruby lasers [69]. In 1969, Van Tran and
Patel produced FIR radiation around 100m by mixing the 10.6m and 9.m bands of
two CO2 lasers in a n-type InSb crystal [70]. These experiments were the forerunners to
Tunable Far Infrared (TuFIR) Spectrometers, which are discussed in more detail in
section 1.2.
More recently two alternative tunable FIR sources have been developed:
1. the free electron laser (FEL),
2. solid state diode FIR lasers.
FEL’s are based on the theory that if an electron is constrained to a circular path it emits
either synchrotron radiation as it accelerates or Bremsstrahlung as it decelerates [71]. In
FIR-FEL’s the velocity of the electron beam is controlled so that the emitted
Bremsstrahlung radiation is channelled into a particular mode of the electromagnetic
field. In addition, the electron beam is perturbed so that the radiation emitted by each
11
CHAPTER ONE INTRODUCTION
electron adds coherently to the existing field. This is achieved by firing a relativistic
electron beam through a periodic static magnetic field (called a wiggler) that effectively
‘pumps’ the electrons and stimulates emission in a narrow frequency band. This radiation
is emitted parallel to the direction of the electron beam and can be tuned by changing the
wiggler period or the beam velocity. At long wavelengths, interactions between particles
in the beam are used to enhance the gain: this is known as the Raman regime. FIR-FEL’s
typically use pulse line accelerators with beam energies up to 10MeV and 3kA current.
The wiggler fields have periods on the order of a few cm and use field strengths around
0.1T [71]. Deacon et al first reported FIR-FEL operation in 1977 [72]. FIR-FEL’s are
built at national accelerator facilities, and operate between 100 and 300m, with peak
pulse output powers of a few hundred W and spectral resolution around 15GHz [73].
Lewellen and co-workers have recently attempted to build a ‘laboratory’ based FIR-FEL,
operating between 80 and 200m and at pulse powers of 200W [74]. Unfortunately, FIRFEL’s are not suitable for high resolution gas phase spectroscopy due to their cost, the
low resulting spectral resolution and the fact that they are only point tunable in 2GHz
steps. To date no specific gas phase studies have been attempted with FIR-FEL’s.
Solid-state lead salt diode lasers have been widely applied in near-IR
spectroscopy but this technology has not been extended below 500cm-1 [75]. Krömer et
al first suggested that FIR radiation would be generated in bulk semiconductors using a
population inversion of ‘hot’ carriers in the conduction and valence bands [76]. In any
semiconductor the valence band lies below the conduction band with the Fermi-level
halfway between them. Above absolute zero, some electrons are thermally promoted into
the conduction band leaving an absence of electrons (called ‘holes’) in the valence band.
At liquid He temperatures only a weak coupling exists between the current carriers and
the lattice structure, creating a free-carrier ‘gas’. The mobility of this ‘gas’ rises rapidly
as its temperature increases so such carriers are called ‘hot’ electrons or ‘hot’ holes. In a
magnetic field, the continuum of levels in the valance and conduction bands breaks up
into a set of discrete energy levels, called Landau levels. At very high strength magnetic
fields cyclotron resonance is induced and the carriers precess around in the magnetic
field at the cyclotron frequency [6]. Tunable FIR solid state lasers are based on ‘light’
and ‘heavy’ mass ‘hot’ holes in p-type Ge. Laser action is only achieved in the presence
of strong, crossed electric (E-) and magnetic (B-) fields. The B-field lifts the degeneracy
12
CHAPTER ONE INTRODUCTION
of the Landau levels: the E-field is used to pump the holes from one Landau level to
another. Laser action is either based on a population inversion between the light and
heavy hole Landau levels, or simulated cyclotron resonance in the light ‘holes’ [77].
Spontaneous cyclotron emission from p-type Ge was first observed in 1972 [78]
but spontaneous emission from light-heavy hole transitions was not observed until 1982
[79]. The Russians pioneered the development of p-type Ge lasers with Ivanov et al
reporting the first cyclotron resonance laser in 1983 [80] and Andronov et al observing
light-heavy hole stimulated emission for the first time in 1984 [81]. The early advances
in both fields are summarised in two recent reviews [77, 82]. Today these lasers are
constructed from high purity p-type Ge parallelepiped ‘crystals’ with highly polished flat
faces. Coating mirrors onto two faces of the crystal increases the laser gain. The sample
has to be immersed in liquid He, at the centre of a superconducting solenoid. Laser
action switches between light-heavy hole emission and cyclotron emission at B-field
strengths above approximately 3 Tesla [83]. The light-heavy hole laser is characterised
by a broadband emission spectrum whereas the cyclotron laser produces a single narrow
line tunable emission. The population inversion is generated by a 1-5kV voltage with a
1sec pulse width. Frequencies ranging from 500GHz-2.5THz have been achieved, at
peak powers up to 500mW [83]. However, the mode spacing in these lasers is 41GHz,
and their spectral resolution only 6GHz [83]. The tuning is complex since both the Eand B-fields have to be adjusted. To date these lasers have only been used to study solidstate FIR spectra and they are not suitable for high-resolution gas phase spectroscopy.
1.2 TuFIR Spectroscopy
TuFIR spectrometers provide a source of broadly tunable, CW, coherent FIR
radiation at zero magnetic field. The technique is based on mixing fixed frequency and
tunable frequency radiation on a non-linear diode, generating partially tunable
‘sidebands’ at FIR frequencies. There are two main advantages to TuFIR spectroscopy
over earlier FIR laser techniques:
1. the resulting spectra are less complex than in LMR/LER so a wider range of
molecular species can be studied,
2. the accuracy to which molecular transition frequencies can be determined has
been improved by two orders of magnitude at zero field.
13