Photofragment Translational Spectrometer Research Articles

A mini-photofragment translational spectrometer with ion velocity map imaging using low voltage acceleration.

A mini time-sliced ion velocity map imaging photofragment translational spectrometer using low voltage acceleration has been constructed. The innovation of this apparatus adopts a relative low voltage (30-150 V) to substitute the traditional high voltage (650-4000 V) to accelerate and focus the fragment ions. The overall length of the flight path is merely 12 cm. There are many advantages for this instrument, such as compact structure, less interference, and easy to operate and control. Low voltage acceleration gives a longer turn-around time to the photofragment ions forming a thicker Newton sphere, which provides sufficient time for slicing. Ion trajectory simulation has been performed for determining the structure dimensions and the operating voltages. The photodissociation and multiphoton ionization of O2 at 224.999 nm is used to calibrate the ion images and examine the overall performance of the new spectrometer. The velocity resolution (Δν/ν) of this spectrometer from O2 photodissociation is about 0.8%, which is better than most previous results using high acceleration voltage. For the case of CF3I dissociation at 277.38 nm, many CF3 vibrational states have been resolved, and the anisotropy parameter has been measured. The application of low voltage acceleration has shown its advantages on the ion velocity map imaging (VMI) apparatus. The miniaturization of the VMI instruments can be realized on the premise of high resolution.

Resolved (v1, v2 = 1) Combination Vibrational States of CF3 Fragments in the Photofragment Translational Spectra of CF3I.

The photodissociation of CF3I → CF3(v1,v2) + I*/I has been investigated at 248, 266, and 277 nm with our high resolution mini-TOF photofragment translational spectrometer. Based on the theoretical calculations of Clary and of Bowman et al., now in this manuscript, we assign 701 cm-1 to the CF symmetric stretch (breathing) ν1 mode, and 1086 cm-1 to the umbrella ν2 mode of the CF3 fragment. In the obtained TOF spectra of I+ from the I* channel, situated in the 701 cm-1 gaps between the original series of (v1, 0) vibrational peaks, a new series of weaker (v1, 1) vibrational peaks are partially resolved. These observed new peaks with 1086 cm-1 ν2 mode excitation have never been reported in previous literature. In the TOF spectra of I+ from the I channel, the new series of (v1, 1) peaks are also partially resolved. However, these spectra of I channel are less satisfactory, because for higher Eavl and higher ET, the higher resolution of PTS is required. The potential energy at the curve crossing point and the excitation of CF3 (v1, 2) and (v1, 3) vibrational states have been also analyzed.

Photodissociation dynamics of ICH2Cl → CH2Cl + I*/I: photofragment translational spectroscopy at 304 and 277 nm

The photodissociation dynamics of ICH2Cl → CH2Cl + I*/I at 304 and 277 nm has been investigated with our mini-TOF photofragment translational spectrometer with a weak acceleration field of <1 V cm(-1). Many peaks are resolved or partially resolved in the TOF spectra and the photofragment translational spectra (PTS) of both the I*((2)P1/2) channel and the I((2)P3/2) channel. These resolved peaks are assigned to the C-Cl stretch vibrational states of the CH2Cl fragment. The rotational energy ER of the CH2Cl fragment is highly excited due to its asymmetric structure. The value of ER/ET is measured to be about 0.71. In the I* channel, the partitioning of the available energy Eavl into the translational energy ET, the rotational energy ER, and the vibrational energy EV for each resolved vibrational state has been calculated.

Photofragment translational spectroscopy at 304 nm from C-H symmetric stretch excited CH3I [v 1 = 1

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I [v 1 = 1, v 2 = 0] (v 1 denotes the C-H symmetric stretch mode, and v 2 denotes the umbrella mode) is studied with our simple photofragment translational spectrometer. An IR laser is used to excite the ground state CH3I [0, 0] to the C-H symmetric stretch excited CH3I [1, 0]. With IR laser OFF and ON, the fractions of photofragments CH3 (ν 1, ν 2) from the 304 nm photodissociation of CH3I [1, 0] have been determined through the photofragment translational spectra (PTS) from measuring I and I* and also through the PTS from measuring CH3 (0, 0) (1, 0) (0, 1) and (1, 1). The experimental results show that the C-H symmetric stretch vibration (v 1 = 1) in parent molecules is about 66% retained in the photofragments in the I channel, but only 24% in the I* channel. The populations of photofragments CH3 (0, 2) and (0, 3) are higher than CH3 (0, 0) and (0, 1), showing strong inverted population both in I and I* channels.

Photofragment translational spectroscopy of CH3I at 225 nm—with the high excitation of the symmetric stretch vibration of CH3 fragment

The photodissociation dynamics of CH(3)I at 225 nm is studied on our high resolution mini-TOF photofragment translational spectrometer. The photofragment translational spectra of the I* and the I channels via parallel (∥) and perpendicular (⊥) transitions, i.e., of the four pathways (3)Q(0), (3)Q(0) ← (1)Q(1), (1)Q(1), and (1)Q(1) ← (3)Q(0), are obtained with both the symmetric stretch (ν(1)) and the umbrella (ν(2)) vibrational modes of the CH(3) fragments partially resolved. The strong excitation of the symmetric stretch mode (ν(1)) is revealed in both the I and the I* channels. The branching fractions for the four pathways (0.09 for (3)Q(0), 0.03 for (3)Q(0) ← (1)Q(1), 0.34 for (1)Q(1), and 0.54 for (1)Q(1) ← (3)Q(0)) show that the parallel transition ((3)Q(0) ← X) is the major and the I channel is dominant in the photodissociation of CH(3)I at 225 nm. The curve-crossing probability is found to be 0.86 for (1)Q(1) ← (3)Q(0) but 0.08 for (3)Q(0) ← (1)Q(1).

Photofragment translational spectroscopy of ICl near 304 and 280 nm: Observation of an intense hot band effect

The photodissociation dynamics of ICl has been studied near 304 and 280 nm on a simple miniature time of flight (mini-TOF) photofragment translational spectrometer with a short pulse of a weak acceleration field. An intense hot band effect was observed. Many small peaks were resolved in each photofragment translational spectrum (PTS). Based on simulations, the principal peaks were assigned not only to the different photodissociation channels (1) I + Cl, (2) I + Cl*, (3) I* + Cl, or (4) I* + Cl*, but also to the different chlorine isotopes (35Cl and 37Cl). Moreover, some extra peaks showed the existence of an intense hot band effect from vibrationally excited ICl molecules, though only a few percent of ICl molecules remained in the vibrationally excited states in our supersonic molecular beam. Based on the spectra near 304 nm, the quantum yield Φ of each channel, the curve crossing, and the branching fraction σ from each transition state were determined.

Vibrationally Resolved Photofragment Translational Spectroscopy of CH3I from 277 to 304 nm with Increasing Effect of the Hot Band

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

Ultraviolet photodissociation of C2F5I with a small and simple photofragment translational spectrometer

Photodissociation dynamics of C(2)F(5)I near 280 and 304 nm has been investigated on a small and simple time-of-flight photofragment translational spectrometer (PTS). On this new PTS, the photolyzed and ionized fragments, not accelerated by electric field, travel freely for a short flight path (<50 mm) and are detected by microchannel plates. In the spectra of the I(*)((2)P(1/2)) channel at 281.73 and 304.02 nm, vibrational peaks with spacing of approximately 350 cm(-1) are partially resolved, indicating the preferential excitation of CF(2) wag mode (nu(11)=366 cm(-1)) of C(2)F(5) photofragment. The fraction of the available energy disposed into the internal energy is higher than 50% for both I(*) channel and I channel, showing the high excitation of vibration in the C(2)F(5) fragments. The fragment recoil anisotropy parameter beta(I(*)), determined to be 1.70 at 281.73 nm and 1.64 at 304.02 nm, reveals that I(*) atoms are produced predominantly from the parallel (3)Q(0) <-- N transition. The anisotropy parameter beta(I), determined to be 1.25 at 279.71 nm and 0.88 at 304.67 nm, implies that I atoms are produced from two excited states, i.e., direct dissociation via the perpendicular (3)Q(1) <-- N transition, and indirect dissociation via the parallel (3)Q(0) <-- N transition then curve crossing to the (1)Q(1) potential energy surface. Analysis on the recent studies with vibrational state resolution in the photodissociation of alkyl iodides in the A band reveals that the "symmetric bending" mode on alpha-carbon of alkyl iodides is the preferential vibrational excitation mode, which can be explained by the classic impulsive model.

Photofragment Translational Spectroscopy ofn-C3H7I andi-C3H7I near 280 and 304 nm

The photodissociation dynamics of propyl iodides n-C3H7I and i-C3H7I near 280 and 304 nm has been investigated with our mini-TOF photofragment translational spectrometer. When a single laser is applied for both the photodissociation of parent molecules and the REMPI of I atom photofragments, the TOF spectra of photofragments I*(2P1/2) and I (2P3/2) are obtained at four different wavelengths for these two iodides. For n-C3H7I, some small vibrational peaks are partially resolved (with separation of approximately 522 cm-1, corresponding to the RCH2 deformation frequency of the fragment n-C3H7) at 281.73, 279.71, and 304.67 nm. These results show that the RCH2 deformation is mostly excited. For i-C3H7I, we obtain some partially resolved vibrational peaks (with separation of approximately 352 cm-1, corresponding to the HC(CH3)2 out-of-plane bending frequency of the fragment i-C3H7) at 281.73 nm only. For n-C3H7I, the partitioning values of the available energy Eint/Eavl are 0.48 at 281.73 nm and 0.49 at 304.02 nm for the I* channel, and 0.52 at both 279.71 and 304.67 nm for the I channel. These energy partitioning values are comparable with the previous results at different wavelengths in the literature. For i-C3H7I, the Eint/Eavl values are 0.61 at 281.73 nm, 0.65 at 304.02 nm for the I* channel, and 0.62 at 279.71 nm, 0.49 at 304.67 nm for the I channel. The potential-energy-surface crossing and the beta values have also been discussed.

A mini-TOF photofragment translational spectrometer – photofragmentation of CF 3I at 281.73 nm

Using a very weak electric field to accelerate the photofragment ions, we have constructed a very simple photofragment translational spectrometer with only a 50 mm total flight path, but we can get photofragment translational spectra with high resolution. On this apparatus, we have performed the photodissociation of CF 3I at 281.73 nm, and the same laser will ionize I*( 2P 1/2) via REMPI. Seven well-resolved vibrational peaks were obtained. Six of them are assigned to the umbrella vibrational mode v 2 = 0–5 states of photofragment CF 3. The average internal energy E ¯ int = 1928 cm - 1 is about 21% of the available energy.

High-resolution photofragment translational spectra of the photodissociation of CF 3I at 248 nm

The photodissociation of CF 3I at 248 nm was performed on our high-resolution photofragment translational spectrometer. For further improvement of the resolution, we have taken the following procedures: (1) Kr was used as carrier gas, (2) optimum molecular beam angle θ opt (when V MB⊥ V CM) was selected, (3) the longitudinal molecular beam angle spread was reduced, and (4) the ionization region for photofragments was diminished. The photofragment translational spectra of I * ( 2 P 1/2 ) reveal eight well-resolved vibrational peaks. The highest peak was assigned to be v 2=5 of the ν 2 vibrational mode, and then the dissociation energy would be D 0( C–I)=18 810±200 cm −1=53.8±0.6 kcal/mol.

Fast beam photodissociation of the CH2NO2 radical

The photodissociation of the nitromethyl radical, CH2NO2, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH2NO−2 and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time- and position-sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240–270 nm and are identified as (I) CH2NO2→CH2NO+O and (II) CH2NO2→H2CO+NO. In marked contrast to the ultraviolet photodissociation of CH3NO2, no evidence is found for simple C–N bond fission to give (III) CH2NO2→CH2+NO2. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5–8 kcal/mol, while the distribution for channel (II) peaks at ∼60 kcal/mol. The angular distributions for both channels are largely isotropic. The nature of the electronic excitation and dissociation dynamics are considered at length. The upper state in the electronic transition is assigned to the 1 2B1 state. Results of attempts to model various aspects of the dissociation dynamics as statistical processes on the ground state surface indicate this mechanism is very unlikely. Instead, both dissociation channels are believed to occur primarily on excited state surfaces, and mechanisms for these processes are proposed.