[Show abstract][Hide abstract] ABSTRACT: The absolute density of SD radicals in a supersonic jet has been measured
down to $(1.1\pm0.1)\times10^5$ cm$^{-3}$ in a modestly specified apparatus
that uses a cross-correlated combination of cavity ring-down and laser-induced
fluorescence detection. Such a density corresponds to $215\pm21$ molecules in
the probe volume at any given time. The minimum detectable absorption
coefficient was quantum-noise-limited and measured to be
$(7.9\pm0.6)\times10^{-11}$ cm$^{-1}$, in 200 s of acquisition time,
corresponding to a noise-equivalent absorption sensitivity for the apparatus of
$(1.6\pm0.1)\times10^{-9}$ cm$^{-1}$ Hz$^{-1/2}$.
[Show abstract][Hide abstract] ABSTRACT: We describe a novel experimental setup that combines the advantages of both
laser-induced fluorescence and cavity ring-down techniques. The simultaneous
and correlated measurement of the ring-down and fluorescence signals yields
absolute absorption coefficients for the fluorescence measurement. The combined
measurement is conducted with the same sample in a single, pulsed laser beam.
The fluorescence measurement extends the dynamic range of a stand-alone cavity
ring-down setup from typically three to at least six orders of magnitude. The
presence of the cavity improves the quality of the signal, in particular the
signal-to-noise ratio. The methodology, dubbed cavity-enhanced laser-induced
fluorescence (CELIF), is developed and rigorously tested against the
spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density
measurements in a cell. We outline how the method can be utilised to determine
absolute quantities: absorption cross sections, sample densities and
fluorescence quantum yields.
[Show abstract][Hide abstract] ABSTRACT: The production of a translationally cold (T < 1 K) sample of bromine atoms with estimated densities of up to 10(8) cm(-3) using photodissociation is presented. A molecular beam of Br(2) seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms(-1), corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source.
[Show abstract][Hide abstract] ABSTRACT: We have demonstrated a new, accessible and economical technique, dubbed
photostop, for producing high densities of trappable molecules. Direct
measurements are presented of NO molecules produced with a narrow velocity
distribution centered at zero in the laboratory frame. NO2, initially cooled in
a pulsed molecular beam, is photodissociated such that the recoil velocity of
the NO photofragments cancels out the velocity of the beam. NO(X^2Pi_3/2, v=0,
J=1.5) molecules are observed up to 10 mircoseconds after the dissociation
event in the probe volume at an estimated density of 1E7 cm-3 per quantum state
and at a translational temperature of 1.6 K. Through the choice of suitable
precursors, photostop has the potential to extend the list atoms and molecules
that can be slowed or trapped. It should be possible to accumulate density in a
trap through consecutive loading of multiple pulses.
[Show abstract][Hide abstract] ABSTRACT: A method to reconstruct full three-dimensional photofragment distributions from their two-dimensional (2D) projection onto a detection plane is presented, for processes in which the expanding Newton sphere has cylindrical symmetry around an axis parallel to the projection plane. The method is based on: (1) onion-peeling in polar coordinates [Zhao et al., Rev. Sci. Instrum. 73, 3044 (2002)] in which the contribution to the 2D projection from events outside the plane bisecting the Newton sphere are subtracted in polar coordinates at incrementally decreasing radii; and (2) ideas borrowed from the basis set expansion (pBASEX) method in polar coordinates [Garcia et al., Rev. Sci. Instrum. 75, 4989 (2004)], which we use to generate 2D projections at each incremental radius for the subtraction. Our method is as good as the pBASEX method in terms of accuracy, is devoid of centerline noise common to reconstruction methods employing Cartesian coordinates; and it is computationally cheap allowing images to be reconstructed as they are being acquired in a typical imaging experiment.
The Review of scientific instruments 06/2009; 80(5):053104. DOI:10.1063/1.3126527 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photoinduced Fe-to-bpy charge transfer in [{Cp(dppe)Fe}(mu-C[triple bond]CC[triple bond]N){Re(CO)(3)(bpy)}]PF(6) has been observed by ps-TRIR spectroscopy, supported by UV-Vis/IR spectroelectrochemistry and DFT calculations.
Chemical Communications 12/2008; DOI:10.1039/b811357b · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation(,). The conventional wisdom is that collisions with high impact parameters (where the partners only 'graze' each other) are forward scattered and essentially elastic, whereas collisions with low impact parameters transfer a large amount of energy into vibrations and are mainly back scattered. Here we report experimental observations of exactly the opposite behaviour for the simplest and most studied of all neutral-neutral collisions: we find that the inelastic scattering process H + D(2)(v = 0, j = 0, 2) --> H + D(2)(v' = 3, j' = 0, 2, 4, 6, 8) leads dominantly to forward scattering (v and j respectively refer to the vibrational and rotational quantum numbers of the D(2) molecule). Quasi-classical trajectory calculations show that the vibrational excitation is caused by extension, not compression, of the D-D bond through interaction with the passing H atom. However, the H-D interaction never becomes strong enough for capture of the H atom before it departs with diminished kinetic energy; that is, the inelastic scattering process is essentially a frustrated reaction in which the collision typically excites the outward-going half of the H-D-D symmetric stretch before the H-D(2) complex dissociates. We suggest that this 'tug of war' between H and D(2) is a new mechanism for vibrational excitation that should play a role in all neutral-neutral collisions where strong attraction can develop between the collision partners.
[Show abstract][Hide abstract] ABSTRACT: The time-delayed forward scattering mechanism recently identified by Althorpe et al. [Nature (London) 416, 67 (2002)] for the H+D(2)(v=0,j=0)-->HD(v(')=3,j(')=0)+D reaction was analyzed by using quasiclassical trajectory (QCT) methodology. The QCT results were found to match the quantum wavepacket snapshots of Althorpe et al., albeit without the quantum scattering effects. Trajectories were analyzed on the fly to investigate the dynamics of the atoms during the reaction. The dominant reaction mechanism progresses from hard collinear impacts, leading to direct recoil, toward glancing impacts. The increased time required for forward scattered trajectories is due to the rotation of the transient HDD complex. Forward scattered trajectories display symmetric stretch vibrations of the transient HDD complex, a signature of the presence of a resonance, or a quantum bottleneck state.
The Journal of Chemical Physics 05/2008; 128(16):164307. DOI:10.1063/1.2902973 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A quasiclassical trajectory study of the state specific H+D(2)(upsilon = 0,j = 0) --> HD(upsilon' = 0,j' = 0) + D reaction at a collision energy of 1.85 eV (total energy of 2.04 eV) found that the scattering is governed by two unexpected and dominant new mechanisms, and not by direct recoil as is generally assumed. The new mechanisms involve strong interaction with the sloping potential around the conical intersection, an area of the potential energy surface not previously considered to have much effect upon reactive scattering. Initial investigations indicate that more than 50% of reactive scattering could be the result of these new mechanisms at this collision energy. Features in the corresponding quantum mechanical results can be attributed to these new (classical) reaction mechanisms.
The Journal of Chemical Physics 05/2008; 128(16):164306. DOI:10.1063/1.2902972 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The H++D2(v = 0,j = 0)→HD+D+ reaction has been theoretically investigated by means of a time independent exact quantum mechanical approach, a quantum wave packet calculation within an adiabatic centrifugal sudden approximation, a statistical quantum model, and a quasiclassical trajectory calculation. Besides reaction probabilities as a function of collision energy at different values of the total angular momentum, J, special emphasis has been made at two specific collision energies, 0.1 and 0.524 eV. The occurrence of distinctive dynamical behavior at these two energies is analyzed in some detail. An extensive comparison with previous experimental measurements on the Rydberg H atom with D2 molecules has been carried out at the higher collision energy. In particular, the present theoretical results have been employed to perform simulations of the experimental kinetic energy spectra.
The Journal of Chemical Physics 01/2008; 128(1):014304-014304-15. DOI:10.1063/1.2812555 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A recent puzzle in nonadiabatic quantum dynamics is that geometric phase (GP) effects are present in the state-to-state opacity functions of the hydrogen-exchange reaction, but cancel out in the state-to-state integral cross sections (ICSs). Here the authors explain this result by using topology to separate the scattering amplitudes into contributions from Feynman paths that loop in opposite senses around the conical intersection. The clockwise-looping paths pass over one transition state (1-TS) and scatter into positive deflection angles; the counterclockwise-looping paths pass over two transition states (2-TS) and scatter into negative deflection angles. The interference between the 1-TS and 2-TS paths thus integrates to a very small value, which cancels the GP effects in the ICS. Quasiclassical trajectory (QCT) calculations reproduce the scattering of the 1-TS and 2-TS paths into positive and negative deflection angles and show that the 2-TS paths describe a direct insertion mechanism. The inserting atom follows a highly constrained "S-bend" path, which allows it to avoid both the other atoms and the conical intersection and forces the product diatom to scatter into high rotational states. By contrast, the quantum 2-TS paths scatter into a mainly statistical distribution of rotational states, so that the quantum 2-TS total ICS is roughly twice the QCT ICS at 2.3 eV total energy. This suggests that the S-bend constraint is relaxed by tunneling in the quantum system. These findings on H+H(2) suggest that similar cancellations or reductions in GP effects are likely in many other reactions.
The Journal of Chemical Physics 02/2007; 126(4):044317. DOI:10.1063/1.2430708 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The torsional motions of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB), a prototype molecular wire, were studied using cavity ring-down spectroscopy in the first UV absorption band (316-321 nm). The torsional spectrum of 1,4-bis(phenylethynyl)-2,3,5,6-tetradeuteriobenzene was also recorded in the gas phase. Both spectra were successfully simulated using simple cosine potentials to describe the torsional motions. The ground-state barrier to rotation was estimated to be 220-235 cm(-1), which is similar to that of diphenylacetylene (tolane). Complementary DFT calculations were found to overestimate the torsional barrier.
The Journal of Physical Chemistry A 03/2006; 110(6):2114-21. DOI:10.1021/jp054426h · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The crossing of two electronic potential surfaces (a conical intersection) should result in geometric phase effects even for molecular processes confined to the lower surface. However, recent quantum simulations of the hydrogen exchange reaction (H + H2 --> H2 + H) have predicted a cancellation in such effects when product distributions are integrated over all scattering angles. We used a simple topological argument to extract reaction paths with different senses from a nuclear wave function that encircles a conical intersection. In the hydrogen-exchange reaction, these senses correspond to paths that cross one or two transition states. These two sets of paths scatter their products into different regions of space, which causes the cancellation in geometric phase effects. The analysis should generalize to other direct reactions.
[Show abstract][Hide abstract] ABSTRACT: The scattering of highly excited hydrogen Rydberg atoms, H* (n = 36), with deuterium molecules in their rovibrational ground state, D2(v = 0, j = 0), has been investigated at a relative collision energy of 0.53 eV. Time-of-flight distributions of elastically/inelastically scattered H* Rydberg atoms and reactively scattered D* Rydberg atoms have been measured at different laboratory angles. The extracted rovibrationally resolved state distributions of the HD product molecules from reactive collisions resemble closely those reported for the corresponding ion-molecule reaction, H+ + D2 --> HD + D+. This similarity is rationalised using the free electron model which predicts that the Rydberg electron acts as a spectator while the ionic reaction takes place.
[Show abstract][Hide abstract] ABSTRACT: The photodissociations of jet-cooled IBr and Br2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(2P3/2o)+Br(2P3/2o) and Br(2P3/2o)+Br∗(2P1/2o) products from parent molecules in their v″ = 0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v″ = 1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v″ = 0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br2.
The Journal of Chemical Physics 04/2002; 116(14). DOI:10.1063/1.1457439 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H + H2 → H2 + H to occur predominantly through a 'direct recoil' mechanism: the H-H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H2 product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process. Indirect exchange mechanisms involving H3 intermediates have been suggested to occur as well, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H + D2 → HD + D, forward scattering features observed in the product angular distribution have been attributed to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.