Chemical Physics Letters 389 (2004) 348–351
www.elsevier.com/locate/cplett
Pump–probe lifetime measurements on singlet
ungerade states in molecular nitrogen
J.P. Sprengers
a
a,*
, A. Johansson b, A. L’Huillier b, C.-G. Wahlstr€
om b,
B.R. Lewis c, W. Ubachs a
Laser Centre, Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
b
Department of Physics, Lund Institute of Technology, P.O. Box 118, S-221 00 Lund, Sweden
c
Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia
Received 11 February 2004; in final form 26 March 2004
Available online 16 April 2004
Abstract
Excited-state lifetimes for a number of vibronic states of 1 Pu symmetry in molecular nitrogen have been determined experimentally in a laser-based pump–probe scheme using a coherent and tunable extreme ultraviolet source based on harmonic generation. States investigated in 14 N2 were b 1 Pu ðv ¼ 6–9Þ, of valence character, and c3 1 Pu ðv ¼ 1Þ and o3 1 Pu ðv ¼ 1Þ, of Rydberg
character. In the case of b 1 Pu ðv ¼ 7Þ and c3 1 Pu ðv ¼ 1Þ, rotationally dependent predissociation was observed. States studied in 15 N2
01 þ
were b 1 Pu ðv ¼ 5Þ and c3 1 Pu ðv ¼ 1Þ; in addition, lifetimes of the c04 1 Rþ
u ðv ¼ 0; 1Þ and b Ru ðv ¼ 1Þ states were determined. The
lifetimes show a dependence on the vibrational quantum number and isotope.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
The excited states in N2 which are accessible from the
ground state X 1 Rþ
g via dipole-allowed transitions are of
1
singlet ungerade (1 Rþ
u and Pu ) symmetry, with corresponding transition frequencies in the extreme ultraviolet (XUV) spectral region. Absorption by N2 in the
upper layers of the atmosphere shields the Earth’s sur1
face from XUV solar radiation. The 1 Rþ
u and Pu states
in N2 undergo predissociation by coupling with triplet
ungerade states, mainly of 3 Pu symmetry. An understanding of the predissociation mechanism is currently
lacking and, therefore, experimental values for the lifetimes of the singlet ungerade states provide valuable new
information to validate models describing the predissociation process. An understanding of the predissociation
in N2 is also important for knowledge of the radiation
budget and photochemistry in the Earth’s atmosphere
[1].
*
Corresponding author. Fax: +31-20-444-7999.
E-mail address: arjan@nat.vu.nl (J.P. Sprengers).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.03.116
Many line-width and lifetime measurements have
been carried out previously for the main isotopomer
14
N2 [2–17], but the data set is not yet complete and
many disagreements exist. Moreover, no lifetime measurements of the singlet ungerade states in 15 N2 have
been reported previously. In the present study, a picosecond laser system was used in a pump–probe configuration to study lifetimes of a number of states:
b 1 Pu ðv ¼ 6–9Þ, c3 1 Pu ðv ¼ 1Þ and o3 1 Pu ðv ¼ 1Þ, in
14
N2 , and b 1 Pu ðv ¼ 5Þ, c3 1 Pu ðv ¼ 1Þ, c04 1 Rþ
u ðv ¼ 0; 1Þ
15
and b0 1 Rþ
N2 . The same experimental
u ðv ¼ 1Þ, in
setup has been used previously to measure lifetimes
1
of the c04 1 Rþ
u ðv ¼ 0–2Þ [16] and b Pu ðv ¼ 1Þ [17] states
in 14 N2 . The lifetimes obtained here are, wherever
possible, compared with literature data from previous
measurements.
2. Experimental method and data analysis
The experimental setup has been described in detail in
[16,17]. Briefly, the frequency-doubled output of a
mode-locked Nd:YAG laser pumped a distributed-
J.P. Sprengers et al. / Chemical Physics Letters 389 (2004) 348–351
Table 1
Lifetimes for some 1 Pu states of
14
v
s (ps)
k (nm)
b 1 Pu
6
7a
380 ± 40
550 ± 40
500 ± 40
320 ± 40
250 ± 40
95 ± 70
650–110
170 ± 30
100 ± 70
650–110
94.91–94.93
94.236
94.246
94.262
94.296
93.51–93.52
92.88–92.90
93.86–93.88
93.90
92.88–92.90
c3 1 Pu
o3 1 Pu
1b
a
Evidence for J -dependent lifetime.
Blended bands.
c
See Section 3.1.
b
15
N2
State
v
s (ps)
c04 1 Rþ
u
0
1
1
5
1a
660 ± 40
530 ± 50
760 ± 60
820 ± 60
1000 ± 100
b0 1 Rþ
u
b 1 Pu
c3 1 Pu
a
Indication of shorter lifetimes at high J -values.
The excitation spectra were simulated by convolving
the estimated bandwidth of the XUV laser with a stick
spectrum representing the relative rotational line
strengths, assuming and accounting for an approximate
rotational temperature between 80 and 150 K, H€
onl1 þ 1 þ
and
R
–
R
transitions
London factors for 1 Pu –1 Rþ
g
u
g
and nuclear-spin statistics on the rotational line
strengths for 14 N2 and 15 N2 , but ignoring possible perturbation effects. The resulting population distributions
are strongly dependent on experimental conditions (e.g.
1
c3 Πu(v=1)
15
N2
1
+
c’4 Σu (v=1)
93.9
93.95
94
94.05
94.1
94.15
Wavelength (nm)
0 1 þ
1 þ
Fig. 1. Spectrum of the c3 1 Pu –X 1 Rþ
g (1,0) and c4 Ru –X Rg (1,0)
bands in 15 N2 . Dashed line: observed spectrum. Full line: calculated
spectrum at T ¼ 130 K. Vertical sticks: line strengths of individual
rotational lines. Perturbations [19] are not included in the simulation.
N2
State
8
9b
1a
Table 2
Lifetimes for some singlet ungerade states of
Intensity (arb.u.)
feedback dye laser (DFDL), and its infrared (IR) output
was amplified in a titanium–sapphire crystal and frequency doubled in a KD*P crystal to produce ultraviolet (UV) radiation. The 8th harmonic of the
fundamental IR beam was obtained by focusing the
temporally and spatially overlapping UV and IR beams
in a pulsed krypton jet. The most probable wave-mixing
process to produce the 8th harmonic was:
mXUV ¼ 3mUV þ 2mIR . The 8th harmonic, selected by a
grating, was refocused in the interaction chamber, where
it crossed a pulsed N2 beam. At resonance, the XUV
pump beam excited the N2 molecules, and a probe
beam, the frequency-tripled 355 nm output of the modelocked Nd:YAG laser, ionised the N2 molecules. The
resulting Nþ
2 ions were accelerated and mass-selected in
a time-of-flight (TOF) drift tube and detected by an
electron multiplier. A 98%-atom 15 N isotopically-enriched gas sample (ISOTEC) was used for the measurements on 15 N2 . Lifetime measurements were carried
out by delaying the probe beam on an optical rail and by
measuring the ion signal over 300 pulses as a function of
time delay. 1 XUV + 1 UV excitation spectra were recorded at zero time delay between pump and probe
pulses. Smooth scanning was accomplished by varying
the temperature of the dye in the DFDL and simultaneously monitoring the wavelength of the fundamental
on a Burleigh wavelength meter.
Important characteristics of the XUV laser were the
bandwidth, DkXUV 0:01 nm, and the pulse duration,
estimated at 100 ps [16,17]. The broad bandwidth meant
that rotational structure could not be fully resolved and
that selected groups of J levels were probed simultaneously during the lifetime measurements. The lifetimes
and their uncertainties were obtained by fitting exponential decay functions to the observed data, having
allowed for the limited temporal resolution of the experimental setup (see [17]). The average lifetimes and
uncertainties obtained from a number of recordings are
listed in Tables 1 and 2, for the levels in 14 N2 and 15 N2 ,
respectively.
349
Rot. lines
Rð0–2Þ
Rð5Þ; Qð2Þ
Rð8Þ; Qð5Þ
Rð11Þ; Qð9Þ; P ð7Þ
Previous results (ps)
350 ± 75 [14], >150 [6]
>150 [6]
>150 [6]
Rð0–13Þ; Qð1–6Þ; P ð2–3Þ
Qð10Þ; P ð6Þ
230c (J ¼ 1) [11]
37c (J ¼ 8) [11]
350
J.P. Sprengers et al. / Chemical Physics Letters 389 (2004) 348–351
1
0
ln(I/I0)
-1
-2
-3
-4
-5
-6
0
1
2
3
Delay time (ns)
Fig. 2. Lifetime decay measurement for the c3 1 Pu ðv ¼ 1Þ level in 15 N2
recorded at the band head. Full line: fit to observed data; dashed line:
time response function of the two laser pulses combined.
nozzle-skimmer distance and gas pulse delay setting)
and do not exactly obey a pure Boltzmann distribution.
From these simulated spectra, the ranges of J levels
probed in the lifetime measurements were estimated. An
example of a simulated spectrum, in comparison with
the experimental spectrum, is shown for the
0 1 þ
1 þ
15
c3 1 Pu –X 1 Rþ
N2
g (1,0) and c4 Ru –X Rg (1,0) bands of
1
in Fig. 1. A lifetime decay curve for the c3 Pu ðv ¼ 1Þ
level of 15 N2 is shown in Fig. 2.
3. Results
3.1. Lifetime measurements for
14
N2
Several levels of 1 Pu symmetry were investigated in
N2 . For bðv ¼ 6Þ, single exponential decays with an
average lifetime of 380 ± 40 ps were observed. This value, obtained with the present Lund time-domain pump–
probe setup, is consistent with the 350 ± 75 ps deduced
from previous line-width experiments with the Amsterdam narrow-band XUV setup [14]. Lifetimes of 350 ps
fall just in the dynamic range, where both experiments
deliver unambiguous and accurate results. It is therefore, reassuring for the consistency of the methods that
good agreement is found for bðv ¼ 6Þ. Our bðv ¼ 6Þ
lifetime is also consistent with [6], where a lower limit of
150 ps was found.
For the bðv ¼ 7Þ state, lifetime measurements were
performed at four wavelength settings, probing different
sets of rotational levels (see Table 1). At the longer
wavelengths, where higher rotational states were probed, a lower signal intensity was obtained; hence the
relative uncertainty is larger. These measurements
demonstrate a strong decrease of excited-state lifetime
for higher rotational quantum numbers in bðv ¼ 7Þ. As
is detailed in Table 1, there is a steady decrease in life-
14
time from 550 ps for J ¼ 1–3, down to 250 ps for
J 9. Since, at most wavelength settings, e- and f parity levels were probed simultaneously, a possible
parity dependence of the lifetime can neither be proven
nor ruled out. Finally, our bðv ¼ 7Þ lifetimes are consistent with the results of [6] which found a lower limit of
150 ps.
Although the b–X (8,0) transition has a small oscillator strength as a result of Rydberg-valence interference
[18], the signal strength was sufficient to perform a
lifetime measurement on bðv ¼ 8Þ. However, the observed lifetime is at the edge of the dynamic range of the
system, on the order of 95 ps. In [6], no line broadening
was observed and their estimated lower limit of 150 ps is
not too far off from our result.
The bðv ¼ 9Þ and o3 ðv ¼ 1Þ levels are overlapped and,
due to the limited wavelength resolution of the XUV
laser, these states could not be resolved from each other.
Observed lifetimes vary from lower than 50 to 110 ps,
giving only an indicative value of the lifetimes for these
two levels. No other lifetime measurements have been
previously reported for these states.
Pump–probe decay transients were recorded for
c3 ðv ¼ 1Þ at three wavelengths. A value of 170 ps was
found at k ¼ 93:86 and 93.88 nm, where many lines pile
up: in this window there is the entire R branch as well as
Qð1–6Þ and P ð2–3Þ. At 93.90 nm, higher-J lines were
probed, centered on Qð10Þ and P ð6Þ. At this wavelength,
the lifetime is too short for an accurate determination.
Nevertheless, a clear shortening of the lifetime is found
for higher J . This result may be compared with the
findings of Kawamoto et al. [11], who measured line
widths in the c–a00 ð1; 0Þ band. These authors found line
widths increasing quadratically with J for both the
e- and f -parity components, with an effective parityaveraged Lorentzian width of 0:023 cm1 full-width at
half-maximum (FWHM) at J ¼ 1, and 0:14 cm1
FWHM at J ¼ 8, corresponding to lifetimes of 230 and
37 ps, respectively. The latter lifetime is too short to
measure accurately with our setup. However, the present
lifetimes, including the observed decrease with increasing J , are in quite good agreement with the results of
Kawamoto et al.
3.2. Lifetime measurements for
15
N2
Two states of 1 Pu symmetry were studied in 15 N2 . A
lifetime of 820 ± 60 ps was found for bðv ¼ 5Þ, a factor
of four higher than the lifetime of this level in 14 N2 [14].
The lifetime of the c3 ðv ¼ 1Þ Rydberg state was also
found to be isotope dependent. In 15 N2 , a lifetime of
1000 ± 100 ps was measured at the band head of the R
branch, as shown in Fig. 2, where a lifetime decay curve
is presented. At higher J levels, bi-exponential decay
curves were observed, with components of 1 ns and
300 ps. Although the observation of short-lived com-
J.P. Sprengers et al. / Chemical Physics Letters 389 (2004) 348–351
ponents underlying long-lived ones is unambiguous, the
spectral assignment of these features in a low-resolution
spectrum is difficult, particularly in view of the overlap
of c3 ðv ¼ 1Þ and c04 ðv ¼ 1Þ (see Fig. 1) and the strong
perturbations in c3 ðv ¼ 1Þ [19].
In addition, three states of 1 Rþ
u symmetry were
studied in 15 N2 . The c04 1 Rþ
u ðv ¼ 0Þ Rydberg state has an
average lifetime of 660 ± 40 ps. Rotational levels up to
J ¼ 11 were probed, but no significant J dependence of
the lifetime was found, contrary to the J dependence
observed for c04 ðv ¼ 0Þ in 14 N2 [16,20]. One pump–probe
delay scan was performed at 95.795 nm, exciting the
Rð7Þ and P ð4–5Þ lines of the b0 1 Rþ
u ðv ¼ 1Þ valence state.
The observed lifetime of this level is somewhat larger,
760 ± 60 ps. The longer lifetime of b0 ðv ¼ 1Þ compared
with c04 ðv ¼ 0Þ, was also found for 14 N2 in a frequencydomain line-width experiment [10]. The average lifetime
of the vibrational level c04 1 Rþ
u ðv ¼ 1Þ was found to be
530 ± 50 ps. The lifetime of this level is isotope dependent; the lifetime in 14 N2 is 330 ± 35 ps [16], significantly
lower.
4. Discussion and conclusions
Direct time-domain pump–probe lifetime measurements have been performed on several 1 Pu states in 14 N2
and also, for the first time, in the isotopomer 15 N2 . In
01 þ
15
N2 , the c04 1 Rþ
u ðv ¼ 0; 1Þ and b Ru ðv ¼ 1Þ states were
also studied. The present lifetime measurements on 1 Pu
and some 1 Rþ
u states of the isotopomers of molecular
nitrogen provide further examples of the erratic dependences on vibrational and rotational quantum numbers
observed in many previous studies [2–17]. Aspects of
such behaviour have been attributed to the effects of
Rydberg-valence interaction. For example, Rydbergvalence interactions between the states of 1 Rþ
u symmetry
have been invoked to explain the behaviour of the
lowest three vibrational levels in the c04 1 Rþ
u state [16].
The experimental lifetimes are also found to be different
for the isotopomers of N2 . The lifetime of b 1 Pu ðv ¼ 7Þ
in 14 N2 is J -dependent: four different sets of J levels were
probed and a decrease of the lifetime with J was found.
Similarly, a J -dependence was found for the
15
c3 1 Pu ðv ¼ 1Þ level in 14 N2 . For c04 1 Rþ
u ðv ¼ 0Þ in N2 , no
J -dependence was observed, contrary to findings in 14 N2
[16,20]. All the present observations of excited-state
lifetimes provide valuable input information for a
comprehensive predissociation model, based on Ryd-
351
berg-valence and singlet-triplet interactions, which is
currently being developed by some of the authors.
Acknowledgements
J.P.S. and W.U. gratefully thank the Lund Laser
Centre for their hospitality. The European Community
supported this work under the Access to Research Infrastructures initiative of the Improving Human Potential Programme, Contract Nos. HPRI-CT-1999-00041
and HPRI-CT-1999-00064. Financial support from the
Molecular Atmospheric Physics (MAP) program of the
Netherlands Foundation for Research of Matter (FOM)
is gratefully acknowledged. P. Cacciani is thanked for
help with the lifetime-fitting code, R. Lang and I. Velchev for assistance during some of the measurements,
and N. Ohashi is thanked for providing the Lorentzian
line-width components from [11].
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