I ran across a PDF of ignition a while back and lost a couple of days to it. My recollection is that they were aiming for atomic mercury, because a monatomic h

I ran across a PDF of ignition a while back and lost a couple of days to it. My recollection is that they were aiming for atomic mercury, because a monatomic heavy propellant was one way to optimize the specific impulse. Not as bad as dimethyl mercury, but still quite toxic.