NorthResearch Group

Ion Imaging

The photodissociation dynamics of benchmark chemcial systems is a window into reactive processes at the molecular

level.  We are particularly interested in answering fundamental questions concerning the predissociation dynamics,

product quantum yields, thermochemistry, stereodyanmics, and fragment angular momentum polarization. Our group is

actively engaged in photodissociation studies of representative systems and jet-cooled radicals of atmospheric relevance

using state-selected ion imaging.  Our preliminary results have already stimulated collaboration with several theoretical

groups and broadened our understanding of reactive pathways including the roaming mechanism. The experiments

provide a stringent test for modern theory and allow assessment of the impact that the photochemistry has on

atmospheric modeling.

Velocity-map ion imaging with state-selective detection is a mature technique, a variant of ion imaging developed by

Chandler and Houston to measure the translational energy distributions of the quantum specified products. The

technique utilizes cylindrical focussing optics to focus ions with common velocity in the x,y plane onto the position

sensitive detector. Analysis of the ion images provides information on the speed and angular distribution of the fragments

and permits determination of the energy partitioning, excited state lifetime, product quantum yields, and nature of the

excited state.

   
We have also recently studied the photodissociation dynamics of jet-cooled carbonyl sulfide (OCS) near 214 nm. Our study extends several previous study of OCS dissociation at longer wavelengths. The ion clouds generated are then either crushed in to detector or center sliced and accumulated for analysis. The measured REMPI spectrum of CO provided permitted determination of the nascent vibrational-rotational state distribution (right). We find a CO vibrational branching ratio of 0.79:0.21 for v=0:v=1, indicating substantially higher vibrational excitation than that observed at slightly longer wavelengths. The CO rotational distribution is bimodal for both v=0 and v=1, although the bimodality is less pronounced than at longer wavelengths. The CO state distributions were confirmed from the analysis of two- color S (1D) images (left). Vector correlations, including rotational alignment, indicate that absorption to both the 21A′ (A) and 11A″ (B) states is important in the lower-j part of the rotational distribution, while only 21A′ state absorption contributes to the upper part; this conclusion is consistent with work at longer wavelengths. Classical trajectory calculations from George McBane (Gand Valley State) including surface hopping reproduce the measured CO rotational distributions and their dependence on wavelength well, though they underestimate the v=1 population. The calculations indicate that the higher-j peak in the rotational distribution arises from molecules that begin on the 21A’ state but make nonadiabatic transitions to the 11A’ (X) state during the dissociation, while the lower-j peak arises from direct photodissociation on either the 21A’ or the 11A” states, as found in previous work. 

The Nitrate Radical

We have recently have studied the photodissociation of the nitrate radical, NO3, an important intermediate in both

statospheric and tropospheric chemical cycles. Photolysis in the visible region yields two product channels, NO2 + O and NO +

O2. Despite the importance of the later channel, the mechanism for its formation was a mystery for decades. Although the

pathway to NO and O2 products was originally thought

to arise from a barrier of a concerted 3-center

transition state, no such transition state at an

accessible energy has been identified.

Ion images of the state selectively ionized NO

fragments resulting from photolysis of a jet-cooled

beam of NO3 at 588 nm are shown to the left. We find

clear evidence for two distinct pathways to the

formation of NO + O2 products.  The dominant

pathway (>70% yield) is characterized by vibrationally

excited O2 (3Σg-, v =5-10) and rotationally cold NO (2Π), while the second pathway is characterized by O2 (3Σg-, v =0-4) and

rotationally hotter NO (2Π) fragments.  The excited O2 vibrational distribution is consistent with the distribution obtained

from the O + NO2 -> NO + O2 intermolecular abstraction reaction. We speculated that the first pathway involved “roaming”

dynamics recently implicated in several systems, most notably CH2O.

The second pathway suggested dissociation via an as yet unidentified three-center transition state. Theoretical work by Xiao,

Maeda and Morokuma proposed origins for the two observed pathways. Their calculations identified roaming-like ONO-O

transition states on the ground and first excited (and optically dark) electronic states. The authors speculated that oxygen

atom roaming occurs on the first excited state, but a conical intersection with the ground state along a roaming trajectory

results in dissociation on both ground and excited state potentials (see below).

Our observation that the two components were associated with different lambda doublet propensities of the NO fragments

(insert), consistent with the orbital symmetries of the two states postulated by Xiao, Maeda and Morokum, confirmed the

multistate mechanism.

 

NorthResearch Group

Ion Imaging

The photodissociation dynamics of benchmark chemcial

systems is a window into reactive processes at the molecular

level.  We are particularly interested in answering

fundamental questions concerning the predissociation

dynamics, product quantum yields, thermochemistry,

stereodyanmics, and fragment angular momentum

polarization. Our group is actively engaged in

photodissociation studies of representative systems and jet-

cooled radicals of atmospheric relevance using state-selected

ion imaging.  Our preliminary results have already stimulated

collaboration with several theoretical groups and broadened

our understanding of reactive pathways including the

roaming mechanism. The experiments provide a stringent

test for modern theory and allow assessment of the impact

that the photochemistry has on atmospheric modeling.

Velocity-map ion imaging with state-selective detection is a

mature technique, a variant of ion imaging developed by

Chandler and Houston to measure the translational energy

distributions of the quantum specified products. The

technique utilizes cylindrical focussing optics to focus ions

with common velocity in the x,y plane onto the position

sensitive detector. Analysis of the ion images provides

information on the speed and angular distribution of the

fragments and permits determination of the energy

partitioning, excited state lifetime, product quantum yields,

and nature of the excited state.

   
We have also recently studied the photodissociation dynamics of jet-cooled carbonyl sulfide (OCS) near 214 nm. Our study extends several previous study of OCS dissociation at longer wavelengths. The ion clouds generated are then either crushed in to detector or center sliced and accumulated for analysis. The measured REMPI spectrum of CO provided permitted determination of the nascent vibrational-rotational state distribution (right). We find a CO vibrational branching ratio of 0.79:0.21 for v=0:v=1, indicating substantially higher vibrational excitation than that observed at slightly longer wavelengths. The CO rotational distribution is bimodal for both v=0 and v=1, although the bimodality is less pronounced than at longer wavelengths. The CO state distributions were confirmed from the analysis of two-color S (1D) images (left). Vector correlations, including rotational alignment, indicate that absorption to both the 21A′ (A) and 11A″ (B) states is important in the lower-j part of the rotational distribution, while only 21A′ state absorption contributes to the upper part; this conclusion is consistent with work at longer wavelengths. Classical trajectory calculations from George McBane (Gand Valley State) including surface hopping reproduce the measured CO rotational distributions and their dependence on wavelength well, though they underestimate the v=1 population. The calculations indicate that the higher-j peak in the rotational distribution arises from molecules that begin on the 21A’ state but make nonadiabatic transitions to the 11A’ (X) state during the dissociation, while the lower-j peak arises from direct photodissociation on either the 21A’ or the 11A” states, as found in previous work. 

The Nitrate Radical

We have recently have studied the photodissociation of the

nitrate radical, NO3, an important intermediate in both

statospheric and tropospheric chemical cycles. Photolysis in

the visible region yields two product channels, NO2 + O and

NO + O2. Despite the importance of the later channel, the

mechanism for its formation was a mystery for decades.

Although the pathway to NO and O2 products was originally

thought to arise from a barrier of a concerted 3-center

transition state, no such transition state at an accessible

energy has been identified.

Ion images of the state selectively ionized NO fragments

resulting from photolysis of a jet-cooled beam of NO3 at 588

nm are shown to the left. We find clear evidence for two

distinct pathways to the formation of NO + O2 products.  The

dominant pathway (>70% yield) is characterized by

vibrationally excited O2 (3Σg-, v =5-10) and rotationally cold NO

(2Π), while the second pathway is characterized by O2 (3Σg-, v

=0-4) and rotationally hotter NO (2Π) fragments.  The excited

O2 vibrational distribution is consistent with the distribution

obtained from the O + NO2 -> NO + O2 intermolecular

abstraction reaction. We speculated that the first pathway

involved “roaming” dynamics recently implicated in several

systems, most notably CH2O.

The second pathway suggested dissociation via an as yet

unidentified three-center transition state. Theoretical work

by Xiao, Maeda and Morokuma proposed origins for the two

observed pathways. Their calculations identified roaming-like

ONO-O transition states on the ground and first excited (and

optically dark) electronic states. The authors speculated that

oxygen atom roaming occurs on the first excited state, but a

conical intersection with the ground state along a roaming

trajectory results in dissociation on both ground and excited

state potentials (see below).

Our observation that the two components were associated

with different lambda doublet propensities of the NO

fragments (insert), consistent with the orbital symmetries of

the two states postulated by Xiao, Maeda and Morokum,

confirmed the multistate mechanism.