  We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the
transition from the spontaneously dimerized spin-liquid phase into the Neel
ordered phase. Two excitation branches: the triplet magnon, and the collective
singlet mode, both become gapless at the transition point. However we find that
the length scales associated with these modes are well separated at the quantum
transition. While in the quantum disordered phase the singlet excitation has
finite spectral weight and reflects the existence of spontaneous dimer order,
near the transition point the size of the singlet bound state grows
exponentially with the correlation length, and hence the quasiparticle residue
is exponentially small. Therefore the critical dynamics remains in the O(3)
universality class in spite of the four gapless modes.
