Existing calculations of nonlinear coupling coefficients for fundament
al and harmonic emission via three-wave interactions are refined and u
sed to calculate the conversion efficiency of Langmuir energy into ele
ctromagnetic waves when the Langmuir waves have the observed bursty fo
rm. Resulting field strengths for harmonic emission are found to be co
nsistent with typical ISEE 3 observations at 1 AU. Fundamental emissio
n at 1 AU is found to proceed only when stimulated by the presence of
a source of ion sound waves. However, it is argued that electrostatic
decay of Langmuir waves can supply the necessary waves provided the dr
iving electron beam is sufficiently fast. Under these conditions, the
predicted fundamental field strengths can account for both the highest
and typical fields observed; they also dominate the predicted harmoni
c fields, consistent with observations. This mechanism is also consist
ent with previous observations that fundamental emission generally occ
urs early in type Ill events, when the beam is fastest. For typical pa
rameters it is shown that neither fundamental nor harmonic emission sa
turates its respective source instability, contrary to previous assump
tions. However, saturation cannot be ruled out under particularly favo
rable conditions.