What is the molecular structure of the fragment ion m/z 161 of rosmarinic acid?
The source you link to says that the spectrum was acquired in negative mode.


  The CID experiment on $\ce{[M - H]-}$ ion at m/z 359.08 identified as rosmarinic acid gave the two main constituents of rosmarinic acid, namely caffeic acid at m/z, 179.0 and the 2-hydroxy derivative of hydrocaffeic acid at m/z
  197.0 as illustrated in Figure 3. Similar pattern of fragmentation of rosmarinic acid during CID analysis has been reported by several authors (17, 25, 26) in analyzing extracts of Lamiaceae spices. 


This means that the parent ion is $\ce{(C18H16O8 - H)-}$, and thus the (hydrated) fragment in question is $\ce{C9H7O4-}$.

The first question in any CID mechanism is whether the fragment arises via charge migration or via charge retention.  I think in this case a charge retention mechanism is pretty likely, because phenolic hydroxyl groups are acidic enough to ionize readily via proton loss.  

The next question is where the loss of water comes from.  It's worth noting that the mass spectrum of even just plain old catechol, $\ce{C6H6O2}$, shows a loss of water in negative-mode CID.  That is, the parent ion $\ce{(C6H6O2 - H)-}$ loses water somehow to form $\ce{(C6H3O)-}$.  I see no reason why the same thing couldn't happen to the rosmarinic acid fragment in question.

It's impossible to say for sure without doing a lot of esoteric spectroscopic characterization of gas-phase ions, or least the synthesis and fragmentation of a series of partially deuterated or $\ce{^{18}O}$-labeled rosmarinic acid isotopologues.  Neither of those is an easy task (far from it), so it's nearly impossible to say for sure what the structure of the fragment is.  

But think a perfectly reasonable proposition is that the ion is the same as whatever the structure of the catechol dehydration fragment is, perhaps a deprotonated hydroxybenzyne or maybe an epoxybenzene anion?  Loss of aromaticity in CID fragmentation not unusual.  In fact, a previous Q&amp;A here on Chemistry StackExchange provides yet another example of this.

High-energy gas-phase ion chemistry is weird.