Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 97–101
High resolution molecular physics studies using a laser-based
tunable XUV source at 107 resolving power
W. Ubachs
Laser Centre, Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
Available online 7 March 2005
Abstract
At the Laser Centre Vrije Universiteit, a tunable and narrowband laser-based source of extreme ultraviolet (XUV) radiation was developed
using non-resonant third harmonic generation (THG) of powerful laser pulses in the ultraviolet range. The laser source is either a commercial or
modified pulsed dye laser (grating-based) or a pulsed-dye-amplifier, which is injection seeded by the output of a continuous wave ring-dye laser
or a Ti:sapphire laser. This system opens the possibility to perform spectroscopic research at wavelengths 70–110 nm at a resolution unattainable
with classical light sources. Particularly in the range 90–110 nm a source bandwidth of 0.01 cm−1 can be employed. Examples of gas-phase
molecular beam spectroscopic studies on diatomic molecules, such as H2 , N2 and CO will be presented. Another feature of Q-switch based
XUV laser sources is the possibility to perform multi-step multi-photon experiments in molecules, uncovering otherwise inaccessible states.
© 2005 Elsevier B.V. All rights reserved.
PACS: 42.65.Ky; 32.30.Jc; 42.72.Bj
Keywords: Extreme ultraviolet; Harmonic generation; Molecular spectroscopy
1. Introduction
2. Instrumentation
With the use of low-order harmonic generation in gaseous
jets, narrowband and tunable extreme ultraviolet (XUV) radiation can be produced, that from a spectroscopic perspective
is superior to all other light sources, including synchrotrons.
Particularly, when Fourier-transform limited pulsed lasers are
used, the bandwidth can be as low as 0.01 cm−1 at wavelengths as short as 58 nm [1]. The development of such
Fourier-transform limited XUV sources was pioneered by
Kung and coworkers [2]. Such systems are based on a single color and non-resonant third harmonic generation (THG)
of near-UV pulses, giving access to the XUV range. Later
Merkt and coworkers developed a system based upon two independently tunable Fourier-transform limited laser systems
in a two-photon resonant scheme for XUV and VUV production [3]. Here a review is given on a XUV laser spectrometer
and its applications in a number of gas-phase spectroscopic
studies.
For a description of the setup of the lasers, the vacuum
equipment with a number of differentially pumped chambers, a piezo-electrically driven pulsed valve providing the
gaseous jet, and the detection scheme, we refer to an earlier
paper [4]. The entire wavelength range 70–110 nm can be accessed when commercial pulsed Nd:YAG-pumped dye lasers
are employed. In case of blue laser dyes, pumping at 355 nm
is required, and frequency doubling in BBO-crystals provide the short-wavelength UV that can directly be frequencytripled to reach the 70–80 nm range. As an example, we refer to the study on the resonance lines in Ne at 73–74 nm,
that were studied in high-resolution [5]. Employing 532 nm
Nd:YAG pumping of yellow and red laser dyes, and subsequent frequency-doubling in KDP-crystals, gives access to
the range 90–110 nm. In these systems, the bandwidth in the
XUV is typically 0.25 cm−1 , so better than what is achievable
at synchrotron stations. With the use of Fourier-transform
limited laser sources another leap in resolution can be made,
down to 0.01 cm−1 [1–3]. Wavelengths shorter than 70 nm at
narrow bandwidth are not really necessary, since at the high
E-mail address: wimu@nat.vu.nl (W. Ubachs).
URL: http://www.nat.vu.nl/∼wimu/.
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.elspec.2005.01.200
98
W. Ubachs / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 97–101
Fig. 2. Setup for monitoring XUV-induced fluorescence at longer wavelengths. The elliptically shaped reflector acts as a light collector. Holes are
made in the elliptical mirror for the XUV-beam to pass. A free gas jet form a
pulsed valve comes in through another opening in the elliptical configuration.
Fig. 1. Efficiency curves for non-resonant THG in three different gases in the
wavelength range covered by DCM dye and subsequent frequency doubling
in KDP. The efficiencies are on a relative scale, but it is estimated that the
peaks corresponds to about 108 –109 photons/pulse.
excitation energies levels in atoms and molecules are subject
to dissociation and (auto)-ionization giving rise to intrinsic
widths in the spectra. Of course the study on the Lamb shift
in He marks an exception [1]. Rupper and Merkt recently reported on a non-resonant frequency-mixing scheme, involving a F2 excimer laser running at 157 nm, thereby producing
wavelengths as short as 61 nm [6].
2.1. Non-resonant tripling efficiencies
Various gases can be used as a non-linear medium for nonresonant THG. In Fig. 1 the harmonic conversion efficiencies
are displayed for the tuning range of DCM dye, for Xe, CO
and C2 H2 . In some cases, a truly oscillatory behavior is found,
that should be accounted for in the spectroscopic application
of the source. It should be noted that such THG-yield curves
depend on a number of parameters, such as the laser intensity
(saturation, depletion and ionization effects in the focus), the
gas density and the exact position with respect to the opening
orifice of the pulsed valve (phase-matching phenomena). One
of the great advantages of two-photon resonant wave-mixing
is that no such strong oscillatory patterns are observed.
Another piece of useful instrumentation is a setup with an
elliptical mirror, through which the XUV beam passes, and
which collect the XUV-induced fluorescence at longer wavelengths. For this setup, shown in Fig. 2 and demonstrated in
the Amsterdam-group, also the two-beam grating splitter of
Ref. [7] was used. This setup was used to monitor simultaneously the absorption spectrum of the c4 1 Σu+ − X 1 Σg+ (0, 0)
band in N2 at 94.6 nm and the fluorescence channels in the
c4 1 Σu+ − a1 Πg system at near-UV wavelengths (see Fig. 3).
From a comparison of such spectra information can be obtained on a rotational-state dependent predissociation in the
c4 1 Σu+ Rydberg state, which is known to interact with an
excited valence state b 1 Σu+ .
3. Results
3.1. Rydberg–valence interaction and predissociation in
CO
The XUV-laser system was applied over the last decade for
a number of high-resolution studies on the excited states of
carbon monoxide, starting with the paper by Levelt et al. [9].
These studies are performed to unravel the complicated interactions between Rydberg and valence states and the predis-
2.2. Direct absorption and XUV-induced fluorescence
Although usually 1 XUV + 1 UV photo-ionization is used
for signal detection in most spectroscopic studies, direct
absorption was demonstrated as well. But those measurements suffer from the rather large statistical noise on the
low-repetition rate XUV-pulses and special measures must
be taken to eliminate the resulting baseline noise. This can
be accomplished by using both 1st and −1st order reflections
of a grating, and use one of the XUV beams as a reference
for beam intensity. Such a method was used by Hinnen et
al. [7] and later by Sommavilla et al. [8]. Both setups were
employed to determine absorption cross-sections, taking advantage of the fact that the source bandwidth is much smaller
than the molecular linewidths.
Fig. 3. Simultaneously recorded XUV absorption spectrum of N2 in the
c4 − X(0, 0) band, and the fluorescence decaying to the a1 Πg state. The
arrow points at a line in the b 1 Σu+ − X 1 Σg+ (1, 0) band, which gives no
fluorescence in the detection window.
W. Ubachs / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 97–101
99
Fig. 4. Recording of a 1 Σ + − 1 Σ + band in CO with an origin at λ =
90.88 nm using 1 XUV + 1 UV photo-ionization detection. The width of
the lines is determined by the predissociation of the upper state. Note the
perturbation in the rotational structure.
Fig. 5. Recording of the bandhead region of the b 1 Πu − X 1 Σg+ (4, 0) band
in 14 N2 . The widths of the lines is fully determined by the upper state predissociation rate and a lifetime of 18 ps is deduced.
sociation behavior of CO; these problems have not yet been
solved. For this work, the bandwidth of the laser-based XUV
sources is an important ingredient. From linewidth measurements on singly rotationally-resolved transitions in the large
number of vibronic band systems accurate values on the rate
of predissociation for each state can be deduced. Some of the
studies were performed with a commercial pulsed dye laser
providing about 0.25 cm−1 bandwidth. In molecular beam
studies with strongly reduced Doppler broadening additional
broadening due to a shortened upper state lifetime can be
extracted with an upper limit of 50 ps. As an example, we
present a hitherto unpublished recording of a 1 Σ + − 1 Σ +
band in CO, centered at λ = 90.88 nm, of which the resolution is limited by predissociation (see Fig. 4). Although the
assignment in terms of vibronic structure of the excited state
is uncertain, the obvious perturbation in the rotational structure could be analyzed in detail. While the deperturbed constants of the excited 1 Σ + state are ν0 = 100, 289.915 cm−1 ,
B = 1.912 cm−1 , an interaction matrix element with a perturber state of Hpert = 17.9 cm−1 can explain the observed
features. It gives rise to an anti-crossing in between J = 14
and 15 levels with perturbing shifts of up to 15 cm−1 . The
deperturbation is unambiguous, since the perturbation is visible both in the P and R branches. The only assumption is that
the perturbation is of a homogeneous (hence J-independent)
nature.
The spectra of Fig. 5 were recorded with the ultranarrowband PDA-based XUV source. It improves the dynamic range for investigating predissociation properties to
excited states living shorter than 500 ps. It was employed
to determine predissociation properties of a large number of
vibronic states [10,11] and further work is in progress, including the investigations on singlet–triplet interactions in
the Rydberg–valence complex in N2 .
3.2. Predissociation in N2
The nitrogen molecule is iso-electronic with CO, and the
Rydberg–valence and predissociation phenomena are equally
complicated in this system. Where the photo-properties of CO
are of importance for the modelling of interstellar clouds, the
spectroscopy and excited state dynamics of N2 bears relevance for the upper layers in the Earth’s atmosphere, where
N2 absorbs the XUV solar flux. And similarly for CO and
N2 the strong dipole-allowed absorption spectra lie in the
range of the extreme ultraviolet. Again line broadening experiments in the molecular beam setup at strongly reduced
Doppler broadening provide insight into the predissociation
properties of the molecule. As an example, we show a recording of the b 1 Πu − X 1 Σg+ (4, 0) band for 14 N2 in Fig. 5.
3.3. Multi-step excitation in H2
In several studies, the XUV source was employed in a
scheme of multi-step excitation to probe otherwise inaccessible electronic states in diatomic molecules. One example is
that of molecular nitrogen, where states of g-symmetry were
probed via initial excitation of the long-lived c4 1 Σu+ v = 0
Rydberg state. In this case, the u ↔ g selection rule requires
excitation through an intermediate state. Through this scheme
a number of hitherto unobserved g states in N2 were discovered recently [12].
In H2 there exists a class of double-well electronic states
that support bound energy levels, which are confined to large
internuclear separation in the molecule. After investigation of
outer well states in the H H̄ 1 Σg+ potential a four laser scheme
was employed to probe levels in the B B̄ 1 Σu+ potential. The
excitation scheme and the relevant potential energy curves are
drawn in Fig. 6. These levels are in the Franck–Condon forbidden region and therefore not accessible from the ground
state. The advantage of performing multi-step-excitation is
also that the first XUV-photon, bridging the gap toward the
first electronically excited state in H2 , acts as a quantum state
selector, simplifying the double-resonance spectrum. Since,
the B 1 Σu+ valence state covers rather large internuclear separations, there is a good Franck–Condon overlap with the
outer well part of H H̄ 1 Σg+ .
The multiple excitation scheme was applied to investigate
also the II 1 Πg shallow well bound by only 200 cm−1 below
the n = 2 dissociation limit in H2 [13]. A series of vibrational levels converging to this n = 2 limit were observed for
all three isotopomers (H2 , HD and D2 ) analyzed within the
framework of the Leroy–Bernstein model. In HD a deviation
was found originating in the distinguishability between the
100
W. Ubachs / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 97–101
Fig. 7. Recording of the P(3) line in the C − X(1, 0) Werner band (left)
and the R(0) line in the B − X(9, 0) Lyman band of H2 . The etalon fringes
and the I2 line are also shown. The line marked with (∗) is the t-hyperfine
component of the P(126) line in the B − X(15, 2) band at 16811.7958 cm−1 .
Fig. 6. The four-laser excitation scheme employed to probe rovibrational
levels in the B̄ outer well state of H2 in the sequence X 1 Σg+ − B 1 Σu+ −
I Πg − B̄ 1 Σu+ [14].
two nuclear particles, changing the long-range R−3 potential
into R−6 , therewith supporting less bound levels [15].
The II 1 Πg state was used as a second intermediate to
probe levels in the first double-well state of u symmetry in
H2 . Fig. 6 shows the excitation scheme involving an XUV
photon, again to bridge the gap to the first electronically excited state, and then two additional photons to reach quantum
states in the B̄ 1 Σu+ outer well. A fourth laser photon serves
to produce signal in the form of H+ ions that are monitored
after a time-of-flight ion detection system. A large number
of rovibrational levels (up to v = 26) were observed [14].
Remarkable features are that these outer well potentials,
both B̄ 1 Σu+ and H̄ 1 Σg+ support long-lived states even above
the ionization potential; this phenomenon is associated with
the high barrier, separating the space of large internuclear
separations R from that of short R. Since ionization must
take place at short R, due to the shape of the H+
2 ground state
potential, ionization is dynamically suppressed by the barrier.
The quest for future research is to find even higher double well-states, which according to predictions should lie at
extremely large internuclear separation. Perhaps that adding
additional photons in a multi-stepwise fashion can bridge the
gap to this Franck–Condon forbidden region [16].
3.4. Precision studies in H2 and the Mp /me ratio
Recently, an extensive study was performed into ultrahigh resolution spectroscopy of the B 1 Σu+ − X 1 Σg+ Lyman
and C 1 Πu − X 1 Σg+ Werner bands in H2 . The narrowband
version of the XUV source was applied to record spectra of
138 lines in the collision-free and Doppler-free conditions
of a skimmed and collimated molecular beam. In Fig. 7 a
recording of one of the lines is shown. The absolute transition frequency is determined by referencing the XUV absorption line to the frequency of the fundamental, which is
measured by simultaneous recording of transmission fringes
of a stabilized etalon and of hyperfine components in the
saturation spectrum of I2 . These I2 lines were calibrated in
our group to the MHz accuracy level [17]. From a careful analysis of contributing errors line positions can be deduced with an uncertainty on the absolute wavelength scale
of λ = 0.000, 005 nm. One of the issues that always should
be taken into account in pulsed-dye-amplification producing narrowband laser pulses is the phenomenon of frequency
chirp, i.e. the frequency excursions during the pulse due to
time-dependent gain in the amplifier medium, which effectively produces a minute shift between the seeding frequency
and the centre of the transient frequency [1].
The transition wavelengths of single rotational lines in the
Lyman and Werner bands are of importance for astrophysics.
In outer space these spectral lines are the strongest molecular absorption features, but in view of the short wavelengths,
the lines cannot be observed with terrestrial telescopes. However, the H2 absorption features at large redshift are observable. Hence, a relatively large set of H2 spectral data exist for
quasars at redshift z around 3. From a comparison with the
accurate laboratory data first of all the redshift values can be
determined to extreme accuracy.
In addition the data can be used to verify whether the
proton-to-electron mass ratio µ = Mp /me has varied on a
cosmological timescale. Each energy level in H2 , with electronic, vibrational and rotational quantum numbers, depends
in a specific way on a possible variation of this ratio µ, in
the same way as each level has a specific isotopic level shift.
Therefore, also all spectral lines are dependent on a possible
variation µ/µ in a specific way; this can be made quantitative in a sensitivity coefficient Ki . On the basis of this
concept spectral lines in quasars can be compared with the
lines observed in the laboratory in the modern epoch. In Fig.
8, such a comparison is made for 80 lines observed in three
different quasars and the most accurate laboratory data, provided by our ultra-high resolution XUV spectroscopy. The
slope of the fitted line represents then a possible variation
of µ/µ or a constraint to any variation. The result is that
µ/µ ≤ −0.5 ± 3.6 × 10−5 at the 2σ level [18]. This signifies that the proton-to-electron mass ratio can be considered
W. Ubachs / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 97–101
101
istaion Netherlands (SRON) are acknowledged for financial
support.
References
Fig. 8. Data of three quasars compared with laboratory data in combined fit.
The line presents the result of the fit, with the slope equalling µ/µ.
constant at the 10−5 level over a period of 12 billion years,
corresponding to the lookback time in the quasars. This result can be further quantified as a constraint on the rate of
variation d/dt(µ/µ) ≤ −0.4 ± 3.0 × 10−15 year−1 .
Acknowledgments
The author wishes to thank Fernando Brandi, Patrice Cacciani, Kjeld Eikema, Urs Hollenstein, Arno de Lange, Josselin Philip, Elmar Reinhold, Arjan Sprengers and Iavor
Velchev for their invaluable contributions to the work presented here. Furthermore, he wishes to thank Profs. K. Baldwin, W. Hogervorst, K.P. Huber, C.A. de Lange, B.R. Lewis,
F. Merkt, L. Tchang-Brillet and L. Wolniewicz for their collaborations. The Netherlands Foundation for Fundamental
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