A velocity-map imaging study of methyl non-resonant multiphoton ionization from the photodissociation of CH3I in the A-band (original) (raw)
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Photofragment imaging: The 266 nm photodissociation of CH3I
Chemical Physics Letters, 1989
We use photofragment imaging to study the internal-state and velocity distributions of methyl fragments following photodissociation of CHJ molecules in a pulsed molecular beam by 266 nm radiation. The methyl fragments are state-selectively ionized via 2+ 1 resonance-enhanced multiphoton ionization (REMPI) through the 3p, Rydberg state. The velocity distribution for a particular internal state of the methyl radical is obtained from the images; this velocity distribution is then used to determine the branching of the methyl iodide into either the ground-state iodine, I(zP3,2), or excited-state iodine I(*Pljl), channel or the selected state of the methyl radical. We find that the branching ratio, I(2P,,,)/I(zP,,,), increases with increasing vibrational excitation in the methyl fragment. In addition, we use a line by line analysis to extract populations from the observed spectra of the 0: band of the 3p,t8 transition of the CHJ fragment. The tit reproduces the observed spectrum and represents conservation of the Kquantum number (spin about the C, axis) upon dissociation. For internally cold parent molecules, the amount of rotational energy about the fragment figure axis is found to be about 8 cm-' and about 106 cm-' for rotational enem perpendicular to the figure axis.
Imaging transient species in the femtosecond A-band photodissociation of CH3I
Chemical Physics, 2009
A nonresonant femtosecond laser pulse centered at 802 nm is used to probe the real time photodissociation dynamics of CH 3 I in the A-band at 267 nm. Using multiphoton ionization with this probe laser pulse and velocity map ion imaging of CH 3 + , we have followed the time evolution of the translational energy and spatial anisotropy of the CH 3 fragment, which in turn has permitted to image the C-I bond breaking from the initial Franck-Condon region up to the final products along the reaction coordinate. Given the temporal width of our pump and probe laser pulses ͑ϳ80 fs͒, a mechanism is proposed by which transient species are probed by simultaneous absorption of pump and probe laser pulses through intermediate Rydberg and ionic states of CH 3 I while the pump and probe pulses overlap in time. This study shows how the combination of femtosecond multiphoton ionization and ion imaging techniques provides an ideal tool to resolve in time the different stages of the bond breaking event in a polyatomic molecule.
Imaging transient species in the femtosecond A-band photodissociation of CH[sub 3]I
The Journal of Chemical Physics, 2009
A nonresonant femtosecond laser pulse centered at 802 nm is used to probe the real time photodissociation dynamics of CH 3 I in the A-band at 267 nm. Using multiphoton ionization with this probe laser pulse and velocity map ion imaging of CH 3 + , we have followed the time evolution of the translational energy and spatial anisotropy of the CH 3 fragment, which in turn has permitted to image the C-I bond breaking from the initial Franck-Condon region up to the final products along the reaction coordinate. Given the temporal width of our pump and probe laser pulses ͑ϳ80 fs͒, a mechanism is proposed by which transient species are probed by simultaneous absorption of pump and probe laser pulses through intermediate Rydberg and ionic states of CH 3 I while the pump and probe pulses overlap in time. This study shows how the combination of femtosecond multiphoton ionization and ion imaging techniques provides an ideal tool to resolve in time the different stages of the bond breaking event in a polyatomic molecule.
Coulomb explosion imaging of CH3I and CH2ClI photodissociation dynamics
The Journal of Chemical Physics, 2018
The photodissociation dynamics of CH 3 I and CH 2 ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH 2 ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.
Femtosecond Transition-State Imaging of theA-Band CH3I Photodissociation
Chemphyschem, 2008
Since the early days of femtosecond transition state spectroscopy, both the clocking of the reaction (on-resonance experiments) and the detection of transient species along the reaction coordinate (off-resonance experiments) have been at the heart of femtochemistry. In the pioneering experiments carried out by Zewail and co-workers, the real time photodissociation of ICN was studied by tuning the probe laser on-resonance to the first electronic excited state of the CN fragment which then emits fluorescence. The resonant probe laser opens up an optical coupling region on the potential energy surface (determined by its bandwidth), which allows the clocking of the reaction from the initial Franck-Condon wave packet to the free fragments in the asymptotic region. However, the beauty of femtochemistry arises from the detection of the transient species between the initial and asymptotic wave packets by tuning the probe laser off-resonance to the free fragment.
Advantage of spatial map ion imaging in the study of large molecule photodissociation
The Journal of Chemical Physics, 2017
The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms ≥ 10). In this study, we used the photodissociation of phenol at the S 1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used. We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imaging or Rydberg atom tagging is used in the photodissociation of large molecules. Finally, we propose a modified velocity map ion imaging technique without the disadvantages of the current velocity map ion imaging technique.
Review of Scientific Instruments, 2002
We report an experimental technique provided to study the full three-dimensional velocity distribution of state-selected products of a chemical process. Time-of-flight mass spectroscopy and resonance enhanced multiphoton ionization combined with a position sensitive detector ͑delay-line anode͒ are employed. The technique has a space resolution of 0.4 mm, a time resolution better than 1 ns, and it provides the possibility to detect several products with a minimal difference between arrival times of 17 ns. One major achievement of the new technique is the possibility to determine the full three-dimensional momentum vectors of a chemical reaction product. This is especially valuable for cases where no symmetry is considered in the process. Second, the high sensitivity of the method allowing to observe single ions enables us to study physical and chemical processes at extremely low densities. Three methods for measuring the temperature of a molecular beam with the technique are demonstrated. A novel result of the present work is the study of angular distribution of NO ions due to electron recoil in the ionization of NO(A 2 ⌺ ϩ ). Finally the advantages of the method are examined by studying the speed distributions of Cl atoms in the photolysis of Cl 2 at 355 nm.
Wave packet study of the methyl iodide photodissociation dynamics in the 266−333 nm wavelength range
The European Physical Journal D, 2013
The effect of changing the temporal width of the pump and probe pulses in the time-resolved photodissociation of CH 3 I in the A-band has been investigated using multisurface nonadiabatic wave packet calculations. The effect is analyzed by examining properties like the photodissociation reaction times and the CH 3 fragment vibrational and rotational distributions, by using four different widths of the pump and probe pulses, namely pulses with full-width-athalf-maximum of 100, 50, 20, and 10 fs. Simulations are carried out for two different excitation wavelengths, 295 and 230 nm, located to the red and to the blue of the maximum of the absorption spectrum, in order to explore possible effects of the excitation wavelength. The reaction times are found to decrease significantly with decreasing pulse temporal width. The times associated with the CH 3 + I*( 2 P 1/2 ) dissociation channels decrease more remarkably than those of the CH 3 +I ( 2 P 3/2 ) channels. The results indicate that for excitation wavelengths located to the blue of the absorption spectrum maximum the effect of changing the pulse width is less pronounced than for wavelengths to the red of the spectrum maximum. On the contrary, the CH 3 vibrational and rotational distributions show little variation upon large changes in the pulse width. The trends found are explained in terms of the changes in the spectral bandwidth of the pulses and of the shape and slope of the absorption spectrum at the different excitation wavelengths.
Time-resolved predissociation of the vibrationless level of the B state of CH3I
Physical Chemistry Chemical Physics, 2011
The predissociation dynamics of the vibrationless level of the first Rydberg state 6s (B 2 E) state of CH 3 I has been studied by femtosecond-resolved velocity map imaging of both the CH 3 and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump-probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH 3 fragment. These observations are made by using (2+1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the 3p z 2 A 2 " state of CH 3 to detect specific vibrational levels of CH 3 . The vibrational branching fractions of the CH 3 are recovered by using the individual vibrationally state-selected CH 3 distributions to fit the full kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or CH 3 fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH 3 fragment and the effect of this alignment on its detection efficiency.
A multiphoton ionization study of the photodissociation dynamics of the S2 state of CH3ONO
The Journal of Chemical Physics, 1989
Two-color (1 + 1) REMPI (resonantly enhanced multiphoton ionization) photoelectron spectroscopy is used to probe the NO photofragments produced by the UV photodissociation of methyl nitrite, i.e., CH 3 0NO + hv ...... CH 3 0NO*(S 2) ...... CH 3 0• (X)+ NO(X, v, J). The photofragments are produced in their ground electronic states but with high rotational and translational energy. NO fragment angular distributions, rotational state distributions, and spatial alignment are determined by photoion and photoelectron detection. The initial state alignment is obtained by the CDAD (circularly dichroic angular distribution) technique for the first time. CDAD measurements for rotational levels with 35.5<J<46.5 result in alignment parameters at the classical high-J limit of ~2 > =-0.4. This alignment is consistent with an "impulsive" dissociation mechanism in which photofragment recoil along the CH 3 0-NO bond imparts substantial rotational angular momentum to the NO molecule resulting in a high-J state distribution and preferential rotation in the plane of dissociation. These measurements clearly establish the utility of the CDAD method for probing chemical processes in which spatial alignment plays an important role. Photoion angular distributions are used to probe correlations between the CH 3 0NO transition dipole moment, NO fragment velocity, and angular momentum. These correlations reveal additional details of the photolysis mechanism.