Science - USA (2020-09-25)

spectrum is shown in fig. S2. Recoiling H 2 ions
were mass selected and then recorded on a
position-sensitive detector using velocity map
imaging ( 26 ). A full experimental description
is provided as supplementary materials.
Figure 1 shows a selection of velocity-mapped
images of H 2 after transforming the raw, two-
dimensional image into a slice of the three-
dimensional distribution using BASEX ( 27 ).
The H 2 (v,j) product state is identified under
each image. Some of the rings in the images
exhibit anisotropy. The origin of this anisotropy
is unclear, is sensitive to the initially excited
rovibrational state of H 2 CO, and will be the
subject of further work. Adjacent to each image

is the resultant H 2 speed distribution, which

exhibits complex structure. Conservation of

energy and linear momentum dictates that this

structure is associated with the internal energy

of the CO cofragment. It is well established that

the roaming reaction in H 2 CO produces CO in

excited vibrational states ( 16 , 28 ); however, the

observed structure does not correspond (solely)

to different vibrational populations. The full set

of images and speed distributions for 50 experi-

ments is provided in fig. S2.

The data shown in the top two rows of Fig. 1

probe H 2 (v= 9,j=3)and(v= 8,j= 4). In both

cases, the high internal energy of H 2 ensures

that the CO cofragment can only be formed in

v= 0. Any structure in the speed distribution

must reflect solely the population distribution

in CO rotational states. Although there is in-

sufficient experimental resolution to resolve

individualjstates, both images clearly show

bimodal speed and, hence,j(CO) distribu-

tions. Underneath the experimental curves,

two model Gaussianjdistributions are shown

as stick spectra, wherejhas been transformed

into the H 2 speed coordinate. The blue and

the red stick spectra represent the lower- and

higher-j(CO) components, respectively. Each

stick spectrum was convolved with an instru-

mental function of ~250 m s−^1 to account for

inherent instrumental resolution and the H 2

SCIENCEsciencemag.org 25 SEPTEMBER 2020•VOL 369 ISSUE 6511 1593

(9,3)

29512.9

H 2 state

(v, j)

Internal energy / cm<^1

Ion image

(8,4)

27538.4

(7,4)

25056.4

(6,3)

22005.6

2000 3000 4000 5000

0.0

0.5

1.0

20 15 10 05

CO(v=0, j)

P

(^ s

)

4000 5000 6000 7000

0.0

0.5

1.0

30 25 20 15 100

CO(v=0, j)

CO(v=1, j=0)

P

(^ s

)

3000 4000 5000 6000 7000 8000 9000

0.0

0.5

1.0 CO(v=0, j)

CO(v=1, j)

45 40 35 30 2520 100

35 30 25 2015100

CO(v=2, j=0)

P

(^ s

)

5000 6000 7000 8000 9000 10000

0.0

0.5

1.0

CO(v=3, j=0)

CO(v=0, j)

CO(v=1, j)

CO(v=2, j)

40 35 30 25 150

H 2 Speed (ms<^1 )

35 30 252015100

35 30 2520 10015

P(

s^

)

P

(^ j

)

0.0

0.5

1.0

0 10 20 30 40

P(

j^

)

0.0

0.5

1.0

0 10 20 30 40

P(

j^ )

0.0

0.5

1.0

0 10 20 30 40

P

(^ j

)

0.0

0.5

1.0

0 10 20 30 40

j (CO)

QCT

H 2 speed distribution CO rotational distribution

Fig. 1. Experimental data for speed distributions of H 2 in (v,j) = (9,3), (8,4), (7,4), and (6,3), respectively.The left column shows the velocity-mapped image,
the center column shows the derived H 2 speed distribution along with a fit to a bimodal Gaussian CO rotational distribution, and the right column shows the fitted
CO(v= 0) rotational distributions and those obtained from corresponding QCT simulations.

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