Microwave and Infrared Spectroscopy of Molecular Ions
Takeshi Oka
Department of Chemistry and Department of Astronomy & Astrophysics
The Enrico Fermi Institute, The University of Chicago
1. The Beginning
Spectroscopy of molecular ions is much more difficult than that of stable
molecules and even free radicals because of the low concentration (~10-6) of molecular
ions that can be produced in plasmas. Extremely high sensitivity is required to detect the
weak absorption. For this reason, until 1970 high resolution spectroscopy of molecular
ions had been limited almost exclusively to emission spectra of electronic transitions in the
visible and ultraviolet region [1]. Because molecular ions are produced abundantly in
astronomical plasmas as a result of ionization by cosmic-ray, stellar radiation, and accelerated
electrons, and because they have long lifetimes as a result of the low density of the medium, ion
spectroscopy has been closely related to astronomical observations both in its inception and
applications. Here I summarize their rotational spectra in the microwave and far infrared (0.1100 cm-1 ) and vibration-rotation spectra in the infrared (300-4000 cm-1).
The first rotational spectrum of a molecular ion came from a serendipitous radio
astronomical discovery by Buhl and Snyder in 1970 [2]. They observed a strong
millimeterwave emission at 89.190 GHz that was unknown in the laboratory and
named it the X-ogen line (Fig. 1). Klemperer correctly speculated [3] that the carrier of the
spectrum was protonated carbon monoxide, HCO+, but it took five years before his
conjecture was confirmed in a laboratory experiment by Woods, et al. [4]. The radio
spectrum of HCO+ (and later HN2+) was also the first spectrum of a protonated ion ever observed
in any spectral region and caught laboratory spectroscopists by surprise revealing a huge void in
our knowledge of molecular ion spectroscopy. The observed abundance and ubiquity of HCO+
in many interstellar dust clouds has also fueled the novel idea by Herbst and Klemperer [5] and
Watson [6], that ion-neutral reactions constitutes the major production mechanism for the many
molecules observed by radio astronomers in the hostile low density and low temperature
environment of interstellar space. Their theory greatly stimulated the laboratory activities of
molecular ion spectroscopy and ion-neutral reaction kinetics, and ab initio calculation of
molecular ions.
Infrared spectroscopy of molecular ions had to wait until the advent of laser
spectroscopy. In 1976, Wing, Lamb, and coworkers [7] invented the ingenious Dopplertuned ion-beam laser-resonance method and applied it to HD+. This method, which was
later applied to HeH+, D3+, H2D+ and 3He4He+, however, was not generally applicable
to ion spectroscopy because it relied on near coincidences between ion absorption and CO
laser lines and its detection method was inefficient (a few per cent variation of the cross
section of beam attenuating collisions by the vibrational excitation of the ion). Carrington
and coworkers [8] have overcome the second drawback by using a mass spectroscopic
detection of predissociated ions and applied the sensitive method to many molecular ions
1
near their dissociation limits as discussed in Section 4.1.
Fig. 1. The X-ogen emission line at 89.190 GHz discovered by Buhl and Snyder [2] toward the Orion
Molecular Cloud and several other star-forming regions. Klemperer [3] correctly conjectured that the line
is the J = 1 → 0 rotational transition of protonated carbon monoxide, HCO+. This discovery initiated the
microwave and infrared spectroscopy of molecular ions.
The general method of molecular ion infrared spectroscopy was introduced for the
spectroscopy of H3+ in 1980 [9] combining a frequency tunable laser radiation source and
a positive column plasma of H2. A stick diagram of the observed highly unconventional
vibration-rotation spectrum is shown in Fig. 2. This development triggered an avalanche of
molecular ion spectroscopy in the 1980’s as mentioned in Section 3.
Fig. 2. A tick diagram of the laboratory infrared spectrum of the ν 2 fundamental band of H3+ [9].
Observed lines are marked with asterisks. Note the atypical spectral pattern without obvious regularity or
symmetry because of the strong vibration-rotation interactions. The observation of this spectrum in 1980
triggered the avalanche of molecular ion spectroscopy in the 1980’s.
This method was made applicable to a wider variety of molecular ions by Saykally’s
invention of the velocity modulation method [10] in 1983 and other modulation methods
discussed in Section 3. 2.
2
In this article, I discuss molecular ions that have been studied by a variety of
experimental methods. Limit of space does not allow me to quote original papers exhaustively.
Readers are referred to the comprehensive compilation of papers on microwave (rotational) and
infrared (vibrational) spectra of free radicals and molecular ions by E. Hirota [11,12].
2. Rotational Spectra
2.1 Microwave Spectroscopy
In addition to the low concentration of ions, there were two inhibitions in
attempting microwave spectroscopy of molecular ions: (1) the spectral lines may be very
broad because of the strong long-range Langevin force between ions and neutrals, and (2)
microwave radiation may be strongly attenuated by plasmas. Woods and collaborators found
neither of them is a serious problem and observed the first laboratory rotational spectra of CO+,
HCO+, and HN2+ in 1975 [13, 4] on the basis of the rotational constant determined from
electronic spectroscopy (CO+) and the results of radio astronomy (HCO+ and HN2+). In the early
80’s when infrared spectroscopy started to provide rotational constants, this method caught fire
and many molecular ions have since been observed. Wisconsin (Woods), Duke (De Lucia and
Herbst), Lille (Bogey, Demuynck and Destombes), and Okazaki (Hirota and Saito) were early
centers of observations. The method was extended into the submillimeter wave region by De
Lucia [14]. De Lucia’s use in 1983 of negative glow discharge extended by a magnetic field has
increased the ion production by two orders of magnitude and greatly facilitated microwave ion
spectroscopy [15].
The following molecular ions have been studied.
ArD+, KrD+, XeD+, CF+, NO+, PO+, SiF+, SiBr+, GeF+, SO+, CO+, ArF+, SH-,
HCO+, HOC+, HOSi+, HCS+, HCNH+, N2H+, HBF+, HCCCNH+, HNCCN+, FN2+, FCO+,
HOCO+, HONN+, HOCS+, ArHCO+, H2D+, H3O+, H3S+, H2Cl+, C2H3+, H2COH+, ArH3+
(Isotopic species are not included in this and the following compilations).
2.2 Far Infrared Spectroscopy
Rotational spectra of light hydride ions and transition with high J values appear in
the far infrared (FIR) region. They were initially studied by the laser magnetic resonance
(LMR) method using non-tunable FIR lasers [16] but the method was limited to
paramagnetic ions. Subsequent introduction of tunable FIR laser sources [17] has made
this method more generally applicable. Colorado (Evenson), Berkeley (Saykally),
Nijmegen (Dymanus and Meerts), and Toyama (Takagi and Matsushima) have been
active in this field. The introduction of an ingenious design of hollow cathode in Nijmegen
influenced subsequent ion spectroscopy [17]. Particularly significant was the systematic
studies of the inversion spectrum of H3O+ by Verhoeve et al. [18]. The following molecular
ions have been studied.
HeH+, NeH+, ArH+, KrH+, OH+, SH+, NH+, FH+, HCl+, HBr+, CO+, OH-,
HCO+, N2H+, H3O+, H2O+
3
3. Vibration-Rotation Spectra
Molecular ion spectroscopy started from electronic spectra and then rotational spectra,
but
vibration-rotation spectroscopy in the infrared has become the most productive general method.
This is because vibrational spectra with their rotational structure are active for all molecules
except for homonuclear diatomics. In contrast, many fundamental molecular ions have no stable
electronically excited state and hence no sharp electronic spectra. In fact no electronic spectrum
of any protonated ion has been observed so far. Protons in protonated ions like H3+, H3O+, NH4+,
CH5+, HCO+, etc. are well bound in the ground electronic state but they leave the system when
the ions are electronically excited. As for rotational spectra, many important fundamental
molecular ions such as H3+, CH3+, C2H2+, NH3+, NH4+, etc. do not have a permanent dipole
moment and hence no rotational spectrum. Also a search for a rotational spectral line is very
time consuming and its observation has been almost always guided either by a radio
astronomical discovery or line positions estimated from vibrational spectroscopy.
3.1 Systematics
Starting from molecular hydrogen as the simplest example, cations are produced
(1) by detaching an electron, or (2) by attaching a proton, and anions are produced (3) by
detaching a proton or (4) by attaching an electron (See Fig. 3)
Fig. 3. Molecular ions produced from H2 by adding and subtracting a proton (p) or an electron (e).
The endothermicity of (1) is the ionization energy (15.4 eV for H2) and the exothermicity of (2)
is the proton affinity (4.4 eV for H2). These two processes are general for any neutral molecule
or atom. I shall call the product of (1), H2+, a primary ion and that of (2), H3+, a protonated ion.
All other cations will be called secondary ions. (3) applies only to molecules containing at least
one hydrogen atom and (4) applies only to molecules with positive electron affinity. Because the
electron affinity of H2 is negative, H2- is not stable. Attachment or detachment of a proton
produces iso-electronic species H3+, H2 and H-, and those of an electron produces isoprotonic
series H2+, H2 and (H2-).
The above systematization can be extended to other species as shown in Fig. 4.
4
Fig. 4. Molecular ions (and radicals) produced from CH4, NH3, and H2O. Infrared spectroscopy allows
us to study all these species systematically.
Infrared spectroscopy can be systematically applied to all species in the figure. CH5+-CH4-CH3-,
NH4+-NH3-NH2-, and H3O+-H2O-OH- are isoelectronic with 10 electrons and, like their united
atom, Ne, they all have singlet ground electronic states. Likewise CH4+-CH3-CH2-, NH3+-NH2NH-, and H2O+-OH are isolectronic to the F atom and have doublet ground states. CH3+-CH2CH-, NH2+-NH, and OH+ are isoelectronic to the O atom and have triplet ground state except for
CH3+. Isoelectronic and isoprotonic pairs like CH4 and NH4+, NH3 and H3O+, CH3 and NH3+,
NH2- and H2O, CH2 and NH2+, etc. have similar structure and dynamical properties. For
example, H3O+ has C3v pyramidal structure like NH3 and has the umbrella inversion motion
although the frequency of inversion is much higher (1650 GHz for H3O+ versus 24 GHz for NH3
) [19]. Although species with 1 to 4 protons have stable equilibrium structures, the 5 protons in
protonated methane, CH5+, are highly fluxional and the observed extremely complicated
spectrum as shown in Fig. 5 has yet to be understood [20].
5
Fig. 5. Stick diagrams of the observed
rovibrational spectra of the C-H stretch
modes of CH3+, CH4 and CH5+. The most
fundamental carbocation CH3+ has well
defined semirigid planar structure with
D3h symmetry and a regular spectrum
[22]. CH5+, on the other hand, shows an
extremely complicated spectrum
reflecting the highly fluxional nature of
the C-H bonds; the five equivalent protons
are swarming around the central carbon
[20]. The understanding of this spectrum
will take a few decades.
Although Fig. 4 contains species with only one heavy atom, it can be readily extended to
species with two heavy atoms C2, N2, O2 and CN, CO, NO, etc. Thus, for example, from
acetylene we obtain the primary ion C2H2+, the protonated ion C2H3+ and the deprotonated ion
C2H- which have all been studied although the detection of C2H- is controversial. Spectroscopy
of protonated acetylene C2H3+, was particularly interesting because its shape was definitively
shown to be non-classical bridged structure and the bond breaking tunnelling motion of the three
protons was demonstrated [21] (see Fig. 6).
Fig. 6. The classical and non-classical structure
of protonated acetylene (left) and conversion
between the two structures by tunnelling of
protons (right) [21]. Observed infrared and
microwave spectra clearly demonstrate that the
non-classical bridged structure is more stable and
that the tunnelling occurs very rapidly (~ µ s in
the ground state and ~ns in the excited state of
the C-H stretch vibration) as has been intuitively
assumed by organic chemists. Such bondbreaking tunnelling motion is special for the
ionic molecule and is not observed in neutral
molecules.
Heavy elements in Fig. 4 are limited to C, N, and O because of my astrophysical interest but they
can be extended to Si, P, S; Ge, As, Se; and Sn, Sb, Te, etc. producing a rich collection of ions.
Most of these molecular ions are yet to be studied spectroscopically.
6
3.2 Experimental Methods
Direct infrared absorption spectroscopy using a variety of plasmas has been most
successful in producing a wealth of information on many molecular ions. Positive
column plasmas in various designs of glass tubes and hollow cathode plasmas in metal
tubes have been used. Fig. 7 shows an example of liquid nitrogen cooled positive column
discharge.
Fig. 7. A liquid-N2 cooled triple jacket plasma tube for positive column discharge used in the Ion Factory
in Chicago (http://fermi.uchicago.edu). A gas mixture is led into the inner plasma tube (inner diameter 12
mm) through 18 pairs of inlets after being cooled by liq. N2 contained in the middle tube (6 cm) and
pumped out through 8 outlets. The outer tube (10 cm) holds vacuum to avoid frosting. The length of the
plasma is ~1.4 m. The bellows and spirals are to ease stress of the pyrex glass tubes caused by the large
temperature difference. The laser radiation is passed through the inner tube cyclically in eight traversals
to gain an extended pathlength for absorption.
Infrared spectra of fundamental primary ions such as NH3+, H2O+, and C2H2+, and
secondary ions such as CH3+, CH2+, and NH2+, were observed in positive column
discharges using He dominated plasmas in Chicago [22,23]. The Penning dissociative
ionization, e.g.,
CH4 + He* → CH3+ + H + e- + He.
capitalizing on the high energy metastable He* (E = 19.8 eV for 23S) has been generally used.
Protonated ions such as H3+, H3O+, NH4+, CH5+, HCNH+C2H3+, H2F+, etc. have also been
studied in Chicago and Berkeley. Amano in Ottawa [24] was particularly successful in applying
7
the hollow cathode plasmas for relatively large protonated ions like HCCCNH+, CH3CNH+,
HOCO+, H2COH+, HOCS+, HONN+ and others on the basis of the general proton transfer
reactions from H3+,
H3+ + X → XH+ + H2.
Saykally’s group was successful [25] in studying negative ions such as NH2-, N3-, NCO-,
NCS-, etc. using condensation reactions in positive columns, eg.,
NH2- + NNO → N3- + H2O.
In order to detect weak molecular ion spectra, a variety of modulation techniques
that are unique to ion spectroscopy have been invented to increase the sensitivity. The
elegant velocity modulation method invented by Saykally [10] has been the most
powerful. This method is based on the high mobility of molecular ions in plasmas (~104
cm2/s V). By applying an AC discharge field on the order of 10 V/cm, the Doppler shift
modulates the line position by an amount comparable to the line width. This effective
frequency modulation is particularly useful for picking up weak ion lines from a bush of
much stronger (x106) neutral lines. It also discriminates spectra of anions from those of
cations. For hollow cathode spectroscopy where the discharge electric field is
perpendicular to the direction of the laser beam, concentration modulation was used
capitalizing on the short lifetime of molecular ions [26]. Using the sensitivity of plasma
chemistry on magnetic field, magnetic modulation method has also been used effectively
[27]. Direct absorption spectroscopy in fast ion beams has also been developed and applied for
the measurement of absolute absorption strengths [28].
For plasmas that do not contain strongly absorbing neutrals, Fourier transform infrared
spectroscopy has been effectively used. The pioneering work on ArH+ emission lines by Brault
and Davis [29] and the extensive work on H3+ by Majewski, McKellar, Watson, and others [30]
are particularly noteworthy.
3.3 Observed Molecular Ions
Molecular ions observed by vibration-rotation spectroscopy are given below.
Protonated Ions: HeH+, NeH+, ArH+, KrH+, XeH+,
HCO+, HOC+, HCS+, HCNH+, HOSi+, HCCCNH+, HN2+, HNCCN+,
H3+, H3O+, H3S+, NH4+, CH5+, CH3CNH+,
H2F+, H2Cl+, HOCO+, HOCS+, HONN+, H2COH+, C2H3+.
Primary ions: HF+, HCl+, HBr+, HI+, CO+, NO+, BrCN+, C2H2+, CO2+, H2O+, NH3+,
Secondary ions: CF+, CCl+, SiH+, SiF+, GeF+, SiCl+, NH+, CH+, SH+,
HBF+, CNC+, HBCl+, HBBr+, CH3+, NH2+, CH2+,
Anions: NH-, OH-, SH-, C2-, Si2-, FHF-, NH2-, N3-, NCO-, NCS-.
8
4. Special Methods
In addition to the direct spectroscopic method, in which variations of the number
if photons are counted, several special techniques have been developed in which number
of charges are counted. Such methods are limited to special groups of ions but have
extremely high sensitivity.
4.1 Ion Beam Spectroscopy Near the Dissociation Limit
Carrington and coworkers have developed the ion-beam laser resonance of Wing
and Lamb [7] into a very sensitive method by introducing a new detection method of
directly counting fragment ions that are generated by radiative dissociation of ions [8].
This method is based on the fact that a significant number of molecular ions in plasmas
are populated in high vibration-rotation states near the dissociation limit. The enormously rich
spectrum of H3+, composed of nearly 27000 lines in the 10 µ m region, is particularly
noteworthy [31, 32]. Understanding the observed H3+ in the high energy levels (~4.3 eV) near
the dissociation limit is still a challenge for theorists [33].
Carrington’s method has been later extended to microwave spectroscopy of
fundamental ions such as H2+, He2+, etc. The method of field ionization has made the
microwave spectroscopy efficient and widely applicable [34]. Although appearing in the
microwave, those spectra correspond to electronic transitions near dissociation limits and
provide a challenge for ab initio theorists.
The following molecular ions have been studied by the method.
HeH+, CH+, H3+, H2+, HD+, D2+, He2+, HeAr+, HeKr+, Ne2+, NeAr+, Ar2+, He...N+, HeH2+
4.2 Infrared Predissociation Spectroscopy of Cluster Ions
The method of infrared predissociation spectroscopy of cluster ions was conceived by Y.
T. Lee and realized in spectroscopy of H3+(H2)n and other cluster ions where the spectra were
greatly broadened by the short lifetime of the excited state as a result of rapid predissociation
[35]. When Maier, Bieske, and colleagues applied the method to H2-HCO+, they found a
rotationally resolved spectrum for the ν 1 vibration, indicating that the predissociation of the
cluster ion in the excited state is slower [36]. Since then this method has grown into a widely
applicable, powerful method to study the structure and dynamics of cluster ions [37]. Dopfer,
Maier, and colleagues extensively used this technique together with ab initio theory to study
intermolecular interactions and microsolvation process in charged complexes [37, 38].
Ion clusters studies by this method in the infrared with rotational resolutions include a
variety of combinations of ions:
HN2+, HCO+, HSiO+, CH3CNH+, H3O+, NH4+, CH5+, OH+, N2+, NH2+, CH3+, SiH3+, C3H3+, Cl-,
Bn-, I-,
and neutrals:
He, Ne, Ar, H2, N2, H2O, NH3, C2H2, CH3, CH3Br.
4.3 Other Methods
9
Photodetachment spectroscopy developed by Lineberger is a widely applicable
technique to study negative ions [39]. High resolution infrared spectra of molecular ions
such as C2-, NH-, OH-, CH2CN-, and HNO- have been observed by this method.
The high spectral resolution attained by the invention of zero kinetic energy
(ZEKE) photoelectron spectroscopy [40] has allowed to observe rotationally resolved
vibrational states of NH3+ and CH4+. (See Chapter 4, PES, TPES, and PFI)
A new method of ion spectroscopy has been invented [41] recently in which the small
endothermicity of the hydrogen abstraction reaction
C2H2+ + H2 → C2H3+ + H
is exploited to conduct spectroscopy by infrared pumping C2H2+ and detecting C2H3+. This
technique may develop into a general method by choosing suitable neutral molecules for
reactions with the ion to be studied.
5. Summary and Outlook
A great many molecular ions have been studied in high resolution by rotational and
rovibrational spectroscopy since the serendipitous radio-astronomical detection of HCO+
in 1970. The fundamental protonated ions such as H3+, HeH+, CH5+, NH4+, H3O+, H2F+,
HCO+, HN2+, HNCH+, C2H3+, HOCO+ and many others whose spectra had not previously
been known in any spectral range have been studied. Infrared spectra of fundamental cations
such as CH2+, CH3+, C2H2+, NH2+, NH3+, H2O+, etc. and anions NH-, OH-, NH2-, FHF-, N3-,
NCO-, etc. have been discovered. The quantum mechanical information on the structure
and intra-molecular dynamics obtained from such studies have enriched the chemistry at
the most fundamental level.
Molecular ions, CH+, CO+, SO+, HCO+, HN2+, HOC+, HCS+, HCNH+, HOCO+, H3O+,
H2COH+, and HC3NH+ have been observed by radio astronomers in interstellar space: they are
all protonated ions except for the first three. The infrared spectrum of the most fundamental
protonated ion, H3+, which plays the pivotal role in interstellar chemistry [3, 4] has been detected
in a wide variety of astronomical objects, including planetary ionospheres [42], dense molecular
clouds [43], and the diffuse interstellar medium [44]. The intense H3+ infrared aurora in Jupiter
is shown in Fig. 8.
The strong H3+ absorption observed toward the Galactic center is shown in Fig. 9.
Laboratory microwave and infrared ion spectroscopy which was initiated by astronomical
observations is now providing abundant results that are useful for astronomical observations.
Although simple protonated ions have been nearly exhaustively studied, protonated
oxygen, HO2+ has defied repeated attempts at detection. The CH stretch band of CH5+ has been
obtained [20], but it will take many years to understand the spectrum. There are other
fundamental cations such as C2H+, HCN+, HNO+, c-C3H3+, etc. and anions CH-, CH2-, C2H-, CN-,
BO-, CH3O-, HO2- and many others that are yet to be studied in high resolution. The method of
high resolution infrared spectroscopy is not readily applicable to heavy ions like benzene ion
10
C6H6+, protonated benzene C6H7+ or even C60+, because of spectral congestion. For obtaining
rotationally resolved spectra of such molecular ions, special techniques such as cooling of ions
[47, 48] and/or sub-Doppler spectroscopy will be needed. Molecular ions so far studied are
composed mostly of elements in the first two rows of periodic table, especially of H, He, C, N,
and O, the five most abundant species in the Universe (making them of astrophysical interest),
but many ions containing Si, P, and S have also been studied. Other ions with heavier elements
can be studied using the same techniques. Such spectra will be of great use for diagnostics of
plasmas in the semiconductor industry.
Fig. 8. An image of the Jupiter H3+ infrared aurora at 3.4
µm obtained at NASA’s IRTF at Mauna Kea [45]. The
bright H3+ emissions at high altitude appear against a
planetary disc darkened by low temperature high
pressure methane absorption in low altitude. A few
million megawatts of power are radiated away from the
auroral zone by the H3+ ion. Such observations enable
planetary scientists to study plasma activities on Jovian
ionospheres from ground based observatories. Magnetic
field lines passing Io, the innermost Galilean satellite
orbiting at a radial distance of 5.95Rj (Rj : radius of
Jupiter), and 30 Rj are shown.
Fig. 9. The R(1, 1)u and R(1, 0) doublet
lines of H3+ at 2726.220 cm-1 and
2725.898 cm-1, respectively observed
toward the nuclear infrared source IRS 3
in the Galactic center. The measured H3+
total column density of 3.5 × 1015 cm-2 is
the highest of all astronomical objects so
far observed.
Spectroscopy of cluster ions mentioned in Section 4.2 is somewhat outside the scope of this
article and not enough justice was done to the great development in the short section. A jewel in
this group, the infrared spectrum of the highly symmetric proton bound water dimer,
H2O·H+·OH2, has yet to be observed.
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