Femtosecond fluorescence depletion spectroscopy of NO2 multiphoton dissociation dynamics

Article abstract of DOI:10.1063/1.480046

Two-color femtosecond pump-probe spectroscopy has been applied to the multiphoton dissociation of NO₂ with a time resolution of ≤350 fs. At laser intensities in the range ?3 × 10¹²-2 × 10¹³ W cm^-2 and a wavelength of 400 nm, NO₂ absorbs three photons before dissociation. Nascent NO A ²Σ⁺ product in its n′ = 0 and 1 vibrational levels is probed by single-photon fluorescence depletion at 800 nm. Analyses of the ultrafast transient behavior at different intensities leads to an estimate of ?600 fs for formation of NO A ²Σ⁺ n′ = 0 and 1.

Full text of DOI:10.1063/1.480046

Femtosecond fluorescence depletion spectroscopy of NO 2 multiphoton dissociation
dynamics
R. B. López-Martens, T. W. Schmidt, and G. Roberts
Citation: The Journal of Chemical Physics 111, 7183 (1999); doi: 10.1063/1.480046
View online: http://dx.doi.org/10.1063/1.480046
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 16
22 OCTOBER 1999
Femtosecond fluorescence depletion spectroscopy of NO2 multiphoton
dissociation dynamics
´
R. B. Lopez-Martens, T. W. Schmidt, and G. Roberts
University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, England
͑Received 29 July 1999; accepted 20 August 1999͒
Two-color femtosecond pump–probe spectroscopy has been applied to the multiphoton dissociation
of NO2 with a time resolution of р350 fs. At laser intensities in the range Ӎ3ϫ1012–2ϫ1013
W cmϪ2 and a wavelength of 400 nm, NO2 absorbs three photons before dissociation. Nascent
NO A 2⌺ϩ product in its n ϭ0 and 1 vibrational levels is probed by single-photon
Ј
fluorescence depletion at 800 nm. Analyses of the ultrafast transient behavior at different
intensities leads to an estimate of р600 fs for formation of NO A 2⌺ϩ n ϭ0 and 1.
Ј
© 1999 American Institute of Physics. ͓S0021-9606͑99͒03040-8͔
The twin characteristics of high brightness and high tem-
poral resolution of femtosecond laser pulses afford the capa-
bility of probing multiphoton absorption, ionization and dis-
sociation dynamics in molecules in real time. At moderate
intensities (IӍ1011–1013 W cmϪ2), these processes can be
understood in terms of a perturbative description of the time-
evolution of wavepackets or classical trajectories over the
molecular potentials accessed by the laser pulses. Pioneering
work by the groups of Zewail1 and Gerber2 aimed to map out
the time dependent dynamics of diatomic and triatomic mol-
ecules subsequent to multiphoton absorption. For I2 and
Na2 , it was possible to pinpoint the extent of compression
and extention of the interatomic distances at which resonant
citation of NO2 to a repulsive potential correlating with NO
C 2⌸ϩO 3P followed by single-photon ionization of NO to
form NOϩ X 1⌺ϩ. In contrast, an earlier kinetic analysis by
Singhal et al.,8 who employed 50 fs laser pulses at 375 nm at
an intensity of 1012 W cmϪ2, suggested that DMI occurred
by single-photon absorption of the parent, dissociation via
the lowest energy pathway and subsequent three-photon ion-
ization of ground-state NO X 2⌸r . A number of other stud-
ies have addressed the mechanism of DMI as a function of
wavelength and pulse width for laser intensities between 1013
9
and 2.5ϫ1014 W cmϪ2
.
In our experiments two-color femtosecond fluorescence
depletion pump–probe spectroscopy has been employed to
investigate multiphoton dissociation of NO2 at 400 nm. In-
tense femtosecond laser pulses at 800 nm were generated by
a regeneratively amplified Ti:sapphire laser ͑Spectra-Physics
Lasers͒. A portion of the beam was frequency doubled in a
0.5 mm BBO crystal to provide pump pulses at ␭1ϭ400 nm
and routed via a delay stage to generate a variable optical
path length. The remainder constituted the probe pulse at
␭2ϭ800 nm and was recombined with the pump by a di-
chroic optic so that both beams were focussed colinearly into
a quartz reaction vessel. The incident pump beam intensity
was varied across the range I1Ӎ3ϫ1012–2ϫ1013 W cmϪ2
by rotating a ␭/2 plate before the BBO crystal, thus reducing
the efficiency of second harmonic generation ͑SHG͒. The
extra probe pulse energy generated at lower SHG efficiencies
was compensated for by another ␭/2 plate and polarizer
placed in the probe beam, thereby maintaining a constant
probe beam intensity of I2ϭ1013 W cmϪ2 in all measure-
ments reported here. Dissociative production of NO in the
multiphoton absorption occurred.1͑c͒,2 Similar work on K2
3
¨
has been reported by Woste and co-workers. At higher in-
tensities (Iу1013 W cmϪ2), the electric vector of the laser
pulse dominates the molecular Hamiltonian such that a de-
scription of the light-matter interaction in terms of laser-
dressed potentials becomes more appropriate. Here a large
number of studies have measured the kinetic energy partion-
ing between dissociating fragments, and have related this
information to novel phenomena such as above-threshold
dissociation and ionization, bond softening and Coulomb
explosions.4
This communication reports our first results on the mul-
tiphoton dissociation dynamics of NO2 . This molecule has
been the subject of a truly enourmous number of spectro-
scopic studies on account of its intense visible absorption
spectrum.5 In the single-photon absorption regime, Wittig
and co-workers6 have employed femtosecond pump–probe
spectroscopy with a time resolution of between 0.5 and 2.1
ps to determine the energy dependence of the dissociation
n ϭ0 and 1 vibrational levels of the 3sA 2⌺ϩ Rydberg state
Ј
3
rate to form ground state NO X 2⌸rϩO P, which proceeds
was monitored by depleting A 2⌺ϩn →X 2⌸ n ␥-band
Ј
Љ
r
via a conical intersection between the adiabatic X 2A1 ground
state and A 2B2 low-lying excited state of NO2 . In the mul-
tiphoton absorption regime, a recent study of electron–ion
coincidence rates by Davies et al.,7 carried out at a wave-
length of 375 nm, pulse width of 100 fs and intensity of
Ӎ1012 W cmϪ2, identified the dominant dissociative multi-
photon ionization ͑DMI͒ pathway as due to three-photon ex-
fluorescence as a function of pump–probe time delay. (n
Ј
and n represent the upper and lower vibrational quantum
Љ
numbers.͒ Based on Franck–Condon arguments and selec-
tion rules, we suggest that the depleting probe transition may
involve B 2⌸ A 2⌺ϩ, but this band system awaits defini-
tive spectroscopic assignment. Fluorescence was collected in
a direction perpendicular to the incident laser beams, fo-
0021-9606/99/111(16)/7183/4/$15.00
7183
© 1999 American Institute of Physics
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´
7184
J. Chem. Phys., Vol. 111, No. 16, 22 October 1999
Lopez-Martens, Schmidt, and Roberts
2
FIG. 1. NO A
⌺
ϩn →X 2⌸ n dispersed fluorescence
Ј
Љ
r
spectra recorded with a spectral resolution of 1 nm: ͑a͒
n ϭ2 spectrum in NO resulting from two-photon ab-
Ј
2
sorption at ␭1ϭ400 nm (A
⌺
ϩn ϭ2 x state n ϭ0
Ј Љ
ϩ2ប␻ ); ͑b͒ n ϭ2, 1, 0 spectrum resulting from mul-
Ј
1
tiphoton dissociation of NO2 at ␭1ϭ400 nm. The laser
intensities for both spectra are I1Ӎ8ϫ1012 W cmϪ2
and I2ϭ1013 W cmϪ2
.
FIG. 2. Femtosecond fluorescence
depletion transients for multiphoton
dissociation of NO2 at different pump
laser intensities at ␭1ϭ400 nm: ͑a͒ for
2
NO A
NO A
⌺
ϩn ϭ0 formation; ͑b͒ for
Ј
2
⌺
ϩn ϭ1 formation. The cross
Ј
correlation trace shown in ͑a͒ has a
FWHM of 350Ϯ30 fs. The probe
pulse at ␭2ϭ800 nm and I2ϭ1013
W cmϪ2 is believed to deplete NO
2
A
⌺
ϩn →X 2⌸ n flurorescence by
Ј
Љ
r
2
single-photon excitation of the B
A
⌸
2
ϩ
⌺
band system ͑see text͒.
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7185
J. Chem. Phys., Vol. 111, No. 16, 22 October 1999
NO2 multiphoton dissociation dynamics
FIG. 3. Graph of the square root of the intensity of NO
2
A
⌺
ϩn ϭ1→X 2⌸ n ϭ0 fluorscence at 216 nm ver-
Ј
Љ
r
sus pump laser energy at ␭1ϭ400 nm, demonstrating
the quadratic dependence of NO A
⌺
ϩn ϭ1 forma-
Ј
2
tion on pump laser intensity.
cussed onto the entrance slit of a monochromator and de-
tected photoelectrically. In carrying out our measurements,
care was taken to ensure that the detected fluorescence re-
sulted from nascent NO A 2⌺ϩ molecules produced by NO2
dissociation: this was achieved by ensuring that the probe
beam was focussed to within the volume of gas irradiated by
the pump. Confirmation of the detection of nascent dissocia-
tion product was provided by comparing fluorescence spectra
of NO2 and NO in the absence of probe light ͑see below͒.
Cross-correlation measurements were made by detecting
sum-frequency light from an aluminum surface on which the
pump and probe pulses were colinearly incident, from which
the temporal resolution of the experiment was determined as
no worse than 400 fs and often as short as 300 fs.
which, as shown below, is not observed, whilst the second
mechanism may be excluded by comparison of Figs. 1͑a͒
and 1͑b͒.
Figure 2 shows examples of the fluorescence from NO
A 2⌺ϩ ϭ0, 1 at different pump–probe time delays and dif-
Ј
ferent pump pulse intensities. Each transient is the average of
four or five individual measurements of the fluorescence
depletion in real time. At large negative time delays (␶DϽ
Ϫ700 fs͒, when the probe pulse precedes the pump, the fluo-
rescence signal is simply determined by multiphoton absorp-
tion of 400 nm NO2 and dissociation to NO A 2⌺ϩ which
subsequently fluoresces over a time scale of some 100 ns. At
large positive time delays (␶DϾ700 fs͒, fluoresence from
nascent NO A 2⌺ϩn ϭ1 product is depleted by the probe. In
Ј
Figure
1
displays the dispersed NO A 2⌺ϩn
the case of NO A 2⌺ϩn ϭ0 formation, however, there is no
Ј
Ј
→X 2⌸ n fluorescence spectrum in the absence of probe la-
depletion of fluorescence. We have calculated Franck–
Condon ͑FC͒ factors for probe–pulse excitation of the
B 2⌸ A 2⌺ϩ transition with a photon energy of 1.55 eV
͑800 nm͒, assuming Morse potentials for both electronic
Љ
r
ser light recorded in samples of NO and NO2 , respectively.
In NO alone, two photons of the pump pulse at ␭1ϭ400 nm
nonresonantly excite the A 2⌺ϩn ϭ2 level only, and Fig.
Ј
1͑a͒ shows the spectrum of A 2⌺ϩn ϭ2→X 2⌸ n transi-
states. For the n ϭ0 vibrational level of the A 2⌺ϩ state, we
Ј
Љ
Ј
r
tions, with the vibrational level assignments given in the fig-
ure. In NO2 , however, the spectrum is much more con-
find a FC factor of only 0.007 21 for the B 2⌸n ϭ12
ٞ
A 2⌺ϩn ϭ0 transition, compared to values of 0.0506 and
Ј
gested, showing lines arising from the n ϭ0 and 1 levels of
0.1440 for the 15 1 and 18 2 transitions, respectively.
Since the transition probabilities are proportional to the
square of the FC factors, the efficiency of depletion is of
Ј
the A 2⌺ϩ state as well as n ϭ2. We attribute the appear-
Ј
ance of emission from NO A 2⌺ϩn ϭ0, 1 as due to produc-
Ј
tion via multiphoton dissociation of NO2 . ͑NO A 2⌺ϩ
n
approximate ratio 1:49:399 for n ϭ0, 1, and 2, respectively.
Ј
Ј
ϭ2 is also energetically accessible at our pump wavelength
We suggest therefore, that the different shapes of the tran-
and intensities, but laser intensity fluctuations have so far
sients for formation of NO A 2⌺ϩn ϭ0 and 1 are simply a
Ј
prevented a quantitative analysis of the NO A 2⌺ϩn ϭ2
reflection of the different probabilities for fluorescence
depletion at ␭2ϭ800 nm. Since near total depletion of fluo-
rescence can be observed, it is further suggested that excita-
tion to the B 2⌸ state facilitates probing by 1ϩ2 REMPI.
In Fig. 2͑b͒, the onset of fluoresence depletion at small
Ј
→X 2⌸ n spectrum in NO to extract the contribution aris-
Љ
r
2
ing from dissociation.͒ We note that at the photon energies
required to produce NO A 2⌺ϩ, dissociation leading to NO
X 2⌸rϩO1D products is also energetically feasible. An early
photochemical study indicates that this exit channel accounts
for 50% of the dissociation yield at wavelengths below 214
nm.10 It is possible to conjecture that production of NO
negative values of ␶ is a result of the still high intensity of
D
the trailing edge of the probe pulse. The decrease in fluores-
cence intensity at small positive ␶ values provides a mea-
D
A 2⌺ϩ in the n ϭ0 and 1 levels may also occur either by
sure of the time required for the dissociation of NO2 . A
careful comparison of the transients in Fig. 2͑b͒ and those
obtained at other intensities with the corresponding data for
depletion of NO A 2⌺ϩn ϭ2→X 2⌸ n ϭ0 fluorescence
Ј
vibrational relaxation of n ϭ2 A 2⌺ϩ molecules or by non-
Ј
resonant excitation of background NO. The first mechanism
would result in identical forms for the transients of fluores-
cence depletion as a function of pump–probe time delay
Ј
Љ
r
͑not shown͒ enables us to set a conservative upper limit of
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´
7186
J. Chem. Phys., Vol. 111, No. 16, 22 October 1999
Lopez-Martens, Schmidt, and Roberts
Ӎ600 fs as the time taken for cleavage of the ON–O bond
and complete separation of the incipient NO A 2⌺ϩ molecule
from the force field of the O 3P atom. This result is in agree-
ment with the sub-500 fs dissociation time reported by
Davies et al. for fragmentation of NO2 to yield NO C 2⌸
following three-photon absorption at ␭1ϭ375 nm.7 From
this result it is possible to speculate that the fragmentation
evolves via a quasibound wavepacket motion as the excited
parent molecule adopts a linear Rydberg configuration prior
to product separation. The enhancement in fluorescence from
posed by Davies et al.,7 who studied NO2 multiphoton dis-
sociation at a photon energy of 3.3 eV ͑375 nm͒ and inten-
sity of 1012 W cmϪ2 by femtosecond photoelectron-photoion
coincidence imaging. Using sub-200 fs laser pulses, the na-
scent dissociation product is identified as Rydberg NO
A 2⌺ϩ in its n ϭ0 and 1 ͑and probably 2͒ vibrational levels.
Ј
From an analysis of the fluorescence depletion on the pump–
probe time delay we estimate the time required for NO2
→NO A 2⌺ϩϩO 3P dissociation as no greater than 600 fs.
A full quantitative understanding of the multiphoton disso-
ciation dynamics of NO2 requires an analysis of the time-
and intensity-dependence of pump-probe photon absorption
by NO2 and NO, and this will be reported in a future
publication.12
R.B.L.-M. thanks E.P.S.R.C. for the award of a research
studentship. T.W.S. acknowledges Churchill College Cam-
bridge for a research studentship and the University of Syd-
ney for an Eleanora Sophia Wood Travelling Scholarship.
We are grateful to E.P.S.R.C., the Isaac Newton Trust and
the Royal Society of London for support of this work
through generous equipment grants.
NO A 2⌺ϩn ϭ0 at short positive time delays is similar to
Ј
effects noted by Zewail and coworkers for multiphoton ion-
ization of NaI1͑a͒ and HgI2 ,1͑b͒ and may be attributed to a
cooperative coherent effect of the temporally overlapped co-
polarized pump and probe pulses resulting in enhanced pro-
duction of NO A 2⌺ϩ. The full-width at half maximum of
the enhancement peak increases from 390Ϯ30 fs to 460
Ϯ30 fs as the pump laser intensity is increased from Ӎ6
ϫ1012 W cmϪ2 to Ӎ1.2ϫ1013 W cmϪ2 as shown in Fig.
2͑a͒.
Figure 3 shows plots of the square root of the NO
A 2⌺ϩn ϭ1→X 2⌸ n ϭ0 fluorescence signal I as a func-
Ј
Љ
r
F
tion of the pump pulse intensity at ␭1ϭ400 nm. The linear
1 See, for example, ͑a͒ A. Materny, J.L. Herek, P. Cong, and A.H. Zewail,
J. Phys. Chem. 98, 3352 ͑1994͒; ͑b͒ S. Pedersen, T. Baumert, and A.H.
Zewail, ibid. 97, 12460 ͑1993͒; ͑c͒ R.M. Bowman, M. Dantus, and A.H.
Zewail, Chem. Phys. Lett. 174, 546 ͑1990͒.
ͱ
dependence of IF on the intensity of the pump pulse is
consistent with absorption of three 400 nm photons by NO2 ,
preparing the molecule 0.5 eV below the ionization thresh-
old, in which absorption of the first photon to the A 2B2 is
2 See, for example, T. Baumert and G. Gerber, Phys. Scr. T72, 53 ͑1997͒;
T. Baumert, R. Thalweiser, V. Weiss, and G. Gerber, Z. Phys. D 28, 37
͑1993͒.
resonantly saturated. Absorption of just two photons at ␭
ϭ400 nm ͑6.2 eV total photon energy͒ does not provide
1
3 R.S. Berry et al., Adv. Chem. Phys. 101, 101 ͑1997͒, and references
therein.
sufficient excitation to access the n ϭ0 and 1 levels of NO
Ј
A 2⌺ϩ through photodissociation. Keldysh parameters11 of
between 4.0 and 18.1 for the range of laser intensities em-
ployed here imply that any competitive ionization processes
in NO2 would be dominated by multiphoton events.
4 Molecules in Laser Fields, edited A.D. Bandrauk ͑Marcel Dekker, New
York, 1994͒, and references therein.
5 See, for example, A. Delon and R. Jost, J. Chem. Phys. 110, 4300 ͑1999͒,
and references therein.
6 P.I. Ionov, I. Bezel, S.I. Ionov, and C. Wittig, Chem. Phys. Lett. 272, 257
͑1997͒; S.I. Ionov et al., J. Chem. Phys. 99, 3420 ͑1993͒; G.A. Brucker,
S.I. Ionov, Y. Chen, and C. Wittig, Chem. Phys. Lett. 194, 301 ͑1992͒.
7 J.A. Davies, J.E. LeClaire, R.E. Continetti, and C.C. Hayden, J. Chem.
Phys. 111, 1 ͑1999͒.
In conclusion, this communication reports results on the
multiphoton absorption and dissociation dynamics of NO2
studied by two-color femtosecond fluorescence depletion
spectroscopy at ␭1ϭ400 nm and ␭2ϭ800 nm. An analysis
of the dependence of fluorescence depletion of the product
NO A 2⌺ϩn →X 2⌸ n transition on the intensity of the
8 R.P. Singhal et al., Chem. Phys. Lett. 253, 81 ͑1996͒.
9 R.P. Singhal et al., Chem. Phys. Lett. 292, 643 ͑1998͒; G. Ravindra Ku-
mar and D. Mathur, ibid. 292, 647 ͑1998͒; K.W.D. Ledingham et al., ibid.
247, 555 ͑1995͒; K. Vijayalakshmi, C.P. Safvan, G. Ravindra Kumar, and
D. Mathur, ibid. 270, 37 ͑1997͒.
Ј
Љ
r
pump pulse leads us to deduce that the dominant dissociation
pathway at I1Ӎ3ϫ1012–2ϫ1013 W cmϪ2 and I2ϭ1013
W cmϪ2 is initiated by absorption of three 3.1 eV ͑400 nm͒
photons. This result lends support to the mechanism pro-
10 W.M. Uselman and E.K.C. Lee, Chem. Phys. Lett. 30, 212 ͑1975͒.
11 L.V. Keldysh, Sov. Phys. JETP 20, 1307 ͑1965͒.
12
´
R.B. Lopez-Martens, T.W. Schmidt, and G. Roberts ͑in preparation͒.
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