CHAPTER ONE INTRODUCTION
Although each TuFIR spectrometer in the world is designed slightly differently from any
other the generation methods fall broadly into two categories:
1. non-linear mixing of a FIR gas laser and a tunable microwave source on a
GaAs Schottky Barrier diode:
 TuFIR   FIR  n   wave
(1.5)
2. non-linear mixing of two CO2 lasers and a microwave source on a diode,
(usually a metal-insulator-metal (MIM) diode):
 TuFIR   tunableCO laser   CO laser   n   wave
2
2
(1.6)
These methods are summarised in figure 1.2.
In 1978, Bićanić et al were the first to successfully generate TuFIR sidebands
from a fixed frequency HCN laser (891GHz) and a Klystron source (90GHz) on a
Schottky barrier diode [84]. They obtained sum- and difference-frequency sidebands and
then recorded the 54,253,3 transition in H2S to a few hundred kHz accuracy. However,
the maximum sideband power was only 100nW! Sideband powers up to 50W were
reported from higher frequency discharge lasers [85]. For spectroscopic applications
these sidebands must be separated from the stronger source signals. Over the last 20
years, many technological advances and design changes have been incorporated into
Bićanić’s original work to improve the sideband separation, output power, and frequency
accuracy.
1.2.1 TuFIR Spectrometers
Fetterman et al made the first improvements to Bićanić’s design very soon after
his original work [86]. The electric discharge HCN laser was replaced with an optically
pumped FIR laser and the diode housing was also changed from a closed to an open
structure. This reduced many of the frequency restrictions imposed on the system by the
limited lasing range of the HCN laser. Consequently, the range of accessible sideband
frequencies increased. The sidebands were separated from the source radiation using a
Michelson type interferometer, with a Melinex beamsplitter, instead of a grating
spectrometer. In 1984 Farhoomand et al were able to generate sidebands with powers up
to 3W, (at least one order of magnitude greater than earlier experiments), by
redesigning many of the spectrometer optics [87]. The mixer diode was positioned at the
14
CHAPTER ONE INTRODUCTION
Method 1
FIR Laser System
line- tunable
GaAs Mixer
Tunable Microwave Source
Klystron
+ multipliers
BWO
Synthesiser
Sideband selection
Fabry- Perot
Diplexer
Interferometer
Grating/ Monochromator
Absorption Cell
Molecular Beam
or
Detector
TuFIR Spectrum
0.01
Int
en
sit 0.00
y
29.4
29.5
Frequency (GHz)
Method 2
Fixed Frequency
CO2 laser
Tunable
CO2 laser
MIM Mixer
Tunable Microwave Source
Synthesiser + multipliers
Detector
Sideband selection
Fabry- Perot
Diplexer
Interferometer
Grating/ Monochromator
Absorption Cell
Molecular Beam
or
TuFIR Spectrum
0.01
Int
en
sit 0.00
y
29.4
29.5
Frequency (GHz)
Figure 1.2: The two types of TuFIR Spectrometer
15
CHAPTER ONE INTRODUCTION
apex of an open corner cube structure and the FIR laser radiation was quasi-optically
focused onto the diode by a whisker antenna (approximately 4 long) as first proposed
by Krautle et al [88]. The sidebands were coupled off the diode in the same fashion and
were separated from the source radiation using a Martin-Pulpett Polarising
Interferometer [21] and a Fabry-Perot Interferometer (FPI). They used the lower
(difference-frequency) and upper (sum-frequency) sidebands, generated from a 93GHz
Klystron and the 693GHz HCOOH FIR laser line, to observe pure rotational transitions
in HDO and H2CO with Doppler limited resolution. Excellent agreement was obtained
between this data and previous results from harmonic generation spectrometers. The
spectral resolution was ultimately limited by the frequency accuracy of the FIR laser. As
in the preceding work, Farhoomand et al found that the lower sideband was at least twice
as intense at the upper sideband. Above 1THz the sideband power dropped off rapidly,
since the whisker antenna was not matched with the incoming FIR radiation, the mixer
efficiency had decreased (from 0.075% at 693GHz to 0.003% at 1839GHz!) and the
detector sensitivity reduced. The current status of this spectrometer is described in more
detail in ref. [89].
Piau et al at the University of Lille replaced the Klystron microwave source with
a 2 to 4GHz microwave synthesiser [90]. This improved both the stability and frequency
accuracy of the sidebands. They also replaced the Martin-Pulpett interferometer with a
rooftop mirror and two wire grids, to minimise the feedback into the FIR laser. This
group used a heterodyne detection scheme instead of the usual InSb or Ge bolometric
detector; some of the FIR radiation was specifically allowed to leak through the
absorption cell and then mixed with the TuFIR radiation at the detector diode. The sumfrequency of these two signals was filtered out and the difference-frequency (essentially
the microwave synthesiser frequency) mixed with a second, phase-matched microwave
signal, shifted 30MHz from the original. The spectrum was obtained from the beat
frequency between the two microwave signals. When one further mixing stage was
added to this heterodyne scheme, they were also able to detect higher order sidebands,
i.e. n=2 or 3 in equation 1.5, using the 692GHz FIR line in HCOOH. The fundamental
sideband power was estimated as a few W, and decreased to a few hundred nW as n
increased. This was due to a decrease in the mixing efficiency in both the mixer and
detector diodes. This detection scheme could not discriminate between the upper and
16
CHAPTER ONE INTRODUCTION
lower sidebands nor determine which sideband was absorbed in the gas cell. To establish
this, the frequency shift of the spectrum was observed as the laser was de-tuned or a FPI
inserted along the optical path. Recently the polarisation system in this spectrometer was
replaced by a complex combination of two Martin-Pulpett Interferometers [91]. The
spectrometer can now be operated in a single sideband mode by adjusting the gridpolariser angles and interferometer mirrors. A Gaussian analysis of the optical system
was used to optimise its performance [92]. Following a number of other minor changes
that were made, sideband powers between 5 and 20W have now been obtained.
Unfortunately, the heterodyne detection scheme has never achieved its full potential, and
this spectrometer is confined to studies of stable molecules.
The most versatile TuFIR spectrometer to date was built by Saykally and coworkers at the Lawrence Berkley Laboratories, California. The system is described in
two lengthy review papers [38, 93] and is based on the design of Farhoomand et al. Their
corner cube arrangement is particularly well engineered: the cube is easily removable so
that mixer diodes can be interchanged according to the spectrometer operating
frequency. The whisker antenna is contacted onto the diode in-situ, and is then tunable,
so its length can be optimised with the FIR laser wavelength. A 2-18GHz microwave
synthesiser is used, together with frequency doublers and tripplers, which increases the
sideband tunablility, and provides a broadband scanning range without it being necessary
to switch FIR laser lines. The microwaves are coupled onto the diode via a coaxial cable
(at low frequencies) or a waveguide (at high frequencies). This coupling is optimised by
adjusting a tunable backshort. They have two detector systems: an InSb detector,
operating in the ‘hot’ electron mode between 300GHz and 1THz, or the Putley mode
between 1 and 2.5THz, and a GeGa photoconductor operating from 2 to 7.5THz. As
TuFIR spectroscopy is detector noise limited this ensures that the spectral sensitivity is
optimised throughout the FIR region. The FIR laser is frequency stabilised giving a
bandwidth of less than 100kHz. Typically, the Doppler widths of spectral lines in the
FIR are a several hundred kHz and the total linewidth is easily increased by pressure
broadening. Therefore, the spectrometer resolution is determined by the intrinsic
linewidth of the absorbing species. A planar jet supersonic expansion source was added
to the Berkeley spectrometer to overcome this problem [94], and in these experiments
the frequency accuracy of the FIR laser ultimately limits the resolution.
17
CHAPTER ONE INTRODUCTION
In the last few years two novel TuFIR designs have emerged, both aiming to
overcome the difficulties associated with separating the weak sidebands from the intense
FIR radiation. Mueller et al designed a layered Si-etalon to transmit almost 100% of the
incident FIR laser radiation but reflect at least 80% of the sideband power [95]. They
obtained 10W of sideband power and only 100nW of FIR power at their detector using
the 1564GHz CH3OH FIR laser line and 10GHz microwaves. The very narrow
bandwidth of these etalons precludes their use in TuFIR spectroscopy. Winnewisser et al
have built a TuFIR spectrometer using a submillimetre BWO (330GHz) as the
‘microwave’ source and an optically pumped FIR ring laser [96]. Both the BWO and the
FIR laser radiation are coupled onto the diode quasi-optically using a grating
monochromator and an ellipsoidal mirror. The grating and optics are arranged so that
only one sideband reaches the absorption cell with almost complete rejection of the FIR
radiation. The sidebands and fundamental are separated by around 300GHz, and can be
tuned over 100GHz with a single laser line. A single mesh filter is used to reject any
stray BWO radiation directed towards the detector. The rotational spectra of a number of
open and closed shell molecules have been measured with this instrument [97] including
SH, S2H, NH, NH2, C2H, and their isotopic analogues. Lamb dip spectra have been
observed from other stable species such as CO and HCl. In these experiments linewidths
of less than 40kHz were obtained, and the transition frequencies were determined to
1kHz accuracy.
Following the early work of VanTran et al [70], Aggarwal et al generated FIR
radiation using CO2 difference-frequency mixing in GaAs [98]. The first successful
results from MIM diode TuFIR difference-frequency spectrometers were reported by
Evenson et al in 1984 [99]. They used two CO2 lasers, set to different ro-vibrational
lasing transitions, to generate sidebands from a W-NiO-Ni MIM diode. At 570GHz they
obtained sideband powers of 200nW, and by re-tuning one of the CO2 lasers they were
able to generate FIR radiation up to 3.5THz. A third CO2 waveguide laser was frequency
offset from the fixed frequency laser and focused onto the MIM diode. The beat
frequency generated by mixing these two signals was then added to the FIR radiation. By
changing this beat frequency the FIR sidebands were tunable by 120MHz. They
scanned the J=54 pure rotational transition in CO. With a number of small
improvements to the same spectrometer, the sideband power was increased to 700nW,
18
CHAPTER ONE INTRODUCTION
and FIR radiation was generated from 300GHz to 6.3THz [100]. The pure rotational
spectrum of OH was recorded to a spectral accuracy of 100kHz.
Evenson et al redesigned their original spectrometer, using 3rd order mixing to
generate FIR radiation and increase the tuning range of each sideband up to 20GHz
[101]. Two CO2 lasers were focused onto a W-Co MIM diode, and then mixed with
radiation from a microwave synthesiser. (Odashima et al have reviewed the FIR power
outputs and 3rd order versus 2nd order mixing response of a number of MIM diodes
[102]). TuFIR radiation was then generated at the difference frequency of the two CO2
lasers plus or minus the microwave frequency (as in equation 1.7 with n=1). The
sideband power was only 200nW. This type of spectrometer was used to measure pure
rotational transitions in CO, HF, and HCl, to provide calibration standards for the FIR.
Evenson et al compared the OH spectra recorded on a TuFIR spectrometer with previous
results from a FIR-LMR spectrometer. They concluded that TuFIR spectroscopy was
around 10 times more accurate than FIR-LMR even though the technique was around
100 times less sensitive [100].
As with ‘method 1’ TuFIR spectrometers, the resolution of TuFIR difference
spectrometers is determined by the intrinsic linewidth of the absorbing species. This
method has three advantages over ‘FIR laser plus microwave’ spectrometers:
1. greater frequency accuracy is possible because the CO2 laser frequencies only
fluctuate by around 15kHz,
2. FIR radiation can be generated up to 6THz, (or even 8THz if one CO2 laser is
replaced with an NH3 laser [103]),
3. it is simple to filter the FIR sidebands from the CO2 laser frequencies.
However, the conversion efficiency of GaAs mixers is much higher than that of MIM
diodes, so the power of TuFIR sidebands produced by method 1 is always at least one
order of magnitude higher. Recently Apollonov et al generated FIR radiation at 3THz
from non-linear mixing of two pulsed CO2 lasers in a ZnGeP2 crystal. The peak pulse
power was around 1W [104]! Viciani et al have also shown that differential detection
systems can improve the sensitivity of difference-frequency TuFIR spectrometers by
almost two orders of magnitude [105].
19
CHAPTER ONE INTRODUCTION
1.2.2 Limitations of TuFIR Spectroscopy
This chapter has already highlighted the many advantages of TuFIR spectroscopy
over other tunable FIR techniques. There are, however, two major disadvantages
associated with TuFIR spectroscopy:
1. attainable power,
2. spectral coverage.
The source of both problems is the same: TuFIR sidebands can only be generated above
the detector noise level if the mixing process in the diode is efficient. This means that the
incident radiation must be intense (high powered) and stable, and diode should respond
rapidly to the incident radiation.
To date, the most powerful sidebands generated from a CW TuFIR spectrometer
were only 50W [93]. With these low signal powers detection is difficult and the spectral
sensitivity (S:N ratio) limited. As the TuFIR sidebands ‘originate’ from an antenna, the
beam is always divergent, and even after collimating or focusing it is not possible to use
multi-pass absorption cells to compensate for the low power levels. In the ‘FIR laser plus
microwaves’ spectrometers, the sidebands are never completely separated from the FIR
laser radiation. Consequently the detector also responds to this signal, reducing is
response to the TuFIR radiation. The novel TuFIR spectrometer designed by
Winnewisser et al does appear to be capable of generating higher sideband powers, and
then clearly discriminating these sidebands from the source signals [96]. The major
drawback of this instrument is its cost: the BWO’s will have to be replaced at regular
intervals to maintain the spectrometer’s performance.
Total coverage of all FIR frequencies is also a problem with FIR spectrometers.
TuFIR difference-frequency spectrometers have been designed with 80% frequency
coverage between 300GHz and 8THz [101]. The ‘FIR laser plus microwaves’
spectrometers can be designed to operate between 300GHz and 4.5THz at up to 60%
coverage [91]. Such instruments are prohibitively expensive to most researchers, since a
number of lasers, diodes, detectors, and microwave multipliers are all required to achieve
such high coverage. Even in a ‘money no object’ situation, TuFIR spectrometers will
never be able to access all FIR frequencies. Ultimately the coverage is limited by the
high frequency cut-off of the mixer diode, and (in method 1) the range of strong FIR
20
CHAPTER ONE INTRODUCTION
laser lines which can be used to generate sidebands. Nevertheless, these problems have
not prevented TuFIR spectroscopy being used in a wide range of molecular studies.
1.2.3 Applications of TuFIR Spectroscopy
Stable molecules, weakly bound molecular complexes, ions, and radials have all
been studied using TuFIR spectrometers. The results fall into three categories:
1. high resolution spectroscopy of novel species, often recorded in the FIR for
the first time,
2. pressure broadening studies of known molecular transitions to determine
pressure shift and broadening parameters of molecular lines,
3. accurate determination of transition frequencies of stable molecules for
calibration standards.
All these results are summarised in table 1.1. Details on many of these results can be
found in three reviews of TuFIR spectroscopy [93, 106, 107].
Many groups have combined other spectroscopic techniques with TuFIR
spectroscopy. Zink et al combined a MIM diode TuFIR difference spectrometer with a
Stark absorption cell for laser Stark spectroscopy [108]. The external E-field lifted the
degeneracy of the rotational energy levels, which helped with the assignment of
molecular transitions, particularly for small polyatomic molecules with internal
rotational degrees of freedom. This technique was used to measure the permanent dipole
moment of 13CH3OH for the first time. Bellini, Inguscio, and co-workers built a similar
instrument at LENS, Italy. They have used it to measure the second-order Stark effect in
H2O2 and determined a number of transition dipole moments for the molecule [109].
They also observed stark spectra from HOCl [110]. The same group attempted subDoppler experiments on CH3OH using an IR-FIR double resonance scheme [111].
Velocity selective excitation was achieved with the TuFIR and CO2 beams counterpropagating the absorption cell. The sub-Doppler curve was therefore superimposed over
the FIR transition. Cazzoli et al observed Lamb Dip spectra using their FPI as the
absorption cell [112]. This system is sideband selective and very sensitive: recently they
recorded Lamb Dip spectra of HCCl [113].
21
CHAPTER ONE INTRODUCTION
METHOD 1
METHOD 2
FIR plus microwaves TuFIR
CO2 Laser difference –frequency
spectrometer
TuFIR spectrometer
CH2=CHCN CF3CCH CH3Br
HNCNH CH3OH H2O2 HO35Cl
13
CD3F CD3F CDF3 CH3CCl3
CH3CF3 CH3CCH H2S OCS
CHF2Cl CH2=CH35Cl CH335Cl
CH337Cl CH2=CH37Cl HCP CH3F
H13CP DCP D13CP CO NH3 AsF3
16
O
18
ArH+ ArD+ HCO+ OH+ OD+
84
KrH+
H3O+ CO+ HN2+
4
O
86
KrH+ ArH+ 4HeH+
HeD+ 3HeH+ 3HeD+
Ar-HCl Ar-DCl Ar2-HCl
Ar-H2O C3 Ar-HDO (NH3)2
Ar-D20 (HCl)2 Ar-NH3 CH4Pure Spectra
H2O H2O-N2 H2O-CO H2O-NH3
H2O-CH4 H2O-C3H8 H2O-C6H6
(D2O)2 (D2O)3 (D2O)4 (D2O)5
(D2O)6 (H2O)2 (H2O)3 (H2O)4
(H2O)5 (H2O)6
NO NH OH CF NH2 HCCl
65
CuH
18
6
OH
65
16
CuD
69
CuH
69
CuD O3
OH KH 7LiH 6LiH 7LiD
LiD HO2 ZnH ZnD MgH
HO35Cl HO37Cl
35
ClO 37ClO
Pressure
Broadening
HCl CHF3
CH3CN O2
NO ClO BrO
7
LiH 6LiH 7LiD 6LiD O3
CO HCl HF
Standards
Table 1.1: A summary of the molecules studied by TuFIR Spectroscopy complied from
the literature. Stable molecules are indicated in black, atoms in yellow, ions in blue
clusters in green and transients in red.
22
CHAPTER ONE INTRODUCTION
Van den Heuvel et al combined their ‘FIR laser plus microwaves’ spectrometer
with a dc-hollow cathode discharge cell. They detected CF [114] and NH [115] using
Zeeman modulation. These were the first measurements of transient species with a FIR
spectrometer. By cooling this discharge cell with liquid N2 they were also able to observe
the first TuFIR spectra of ions, i.e. HCO+, CO+ and HN2+ [116]. Laughlin et al used a
negative glow discharge to generate ArH+ and ArD+. They reported the first ever
determination of the electric dipole moment of a molecular ion [117]. More recently
Matsushima et al have combined velocity modulation techniques with their MIM diode
difference-frequency TuFIR spectrometer. They observed the low-lying pure rotational
transitions of HeH+ and its isotopic analogues for the first time [118].
Saykally and co-workers at the University of California, Berkley, have combined
their TuFIR spectrometer with a planar supersonic jet that generates molecular
complexes at low temperatures. The vibrational energies of the weak bonds in these
clusters lie in the FIR. The very high resolution in the TuFIR spectra enabled them to
resolve the rotational and hyperfine structure of these transitions. Spectroscopic
information from these clusters is used to reconstruct their intermolecular potential
energy surfaces, and contributes to our understanding of hydrogen and Van der Waals
bonding. This system was initially used to study vibration-rotation tunnelling (VRT)
spectra of Ar-H2O (NH3)2 and Ar-NH3 [119]. More recently this instrument has been
used in extensive studies of water clusters, (H2O)n, where
n
ranges from 2 to 6. D2O
analogues have also been studied. This work is summarised in refs. [120-123]. The same
group used an excimer laser to produce C3 clusters by vaporising a carbon rod. They
recorded the 2-bending mode of this molecule [124].
The motivation for such detailed laboratory spectroscopy comes from the
astronomy and atmospheric communities. Transition frequencies and spectral line
parameters from TuFIR spectra are used to assign and evaluate spectroscopic data
obtained from satellite observations and atmospheric remote sensing [22-24]. FIR studies
of planetary atmospheres provide important information on their chemical composition
and evolution [125]. A number of molecules and radicals that are significant to global
warming, or Ozone depletion in Earth’s stratosphere, have pure rotational transitions in
the FIR, e.g. O3, HCl, H2O, OH, ClO. Many TuFIR spectrometers have been dedicated to
23
CHAPTER ONE INTRODUCTION
studies of atmospherically important molecules e.g. LENS [126], Cambridge (this
thesis). Astronomers have only recently commenced observations in the FIR. Between
50 and 500m, they are particularly interested in pure rotational transitions of metal
hydrides, excited state transitions in CO, fine structure transitions in O, C+ and N+, and
ro-vibrational transitions of large molecules with >15 atoms e.g. polycyclic aromatic
hydrocarbons [127]. Many of these molecules can only be observed in the FIR, so
observations in this region make a vital contribution to their understanding of the
chemical processes occurring in the Interstellar medium. Consequently, in 2006 ESA
will launch a dedicated FIR telescope, (FIRST), based on high-resolution heterodyne
techniques and operating between 400GHz and 1THz [128]. The TuFIR spectrometer at
the Institute of Molecular Sciences in Japan has been dedicated to studies of transient
molecules of astronomical interest [129].
1.3 TuFIR Spectroscopy in Cambridge
The Cambridge TuFIR spectrometer is based on non-linear mixing of FIR laser
radiation and microwave radiation on a GaAS Schottky barrier diode (method 1).
Boardman constructed the original spectrometer in 1989 [130]. It has previously been
used to observe pressure broadening in stable molecules, e.g. HCl [131] and radicals e.g.
NO [132], ClO [133]. This thesis focuses on two aspects of TuFIR spectroscopy. Firstly
the Cambridge spectrometer has been developed and re-designed to improve its
resolution and sensitivity. These improvements are discussed in detail in chapter 3.
Secondly the spectrometer has been used for extensive pressure broadening studies of
BrO (chapter 4) and CHF3 (chapter 5).
24